My prefix is pretty clever and funny
My infix can sit, though not on his ass,
My suffix is owned by someone,
though not by me, by a bonny lass.

What am I?

• 'Tis inevitable that some day, Riley Riddles will fade away... – Mr Pie Sep 13 '18 at 11:54
• probably,but not anytime soon though :P – Shahriar Mahmud Sajid Sep 13 '18 at 11:57

Using @Cashbee's Prefix, are you:

Withers

Prefix:

WIT

Infix:

From @Cashbee: 'IT' people have desk jobs so they can sit all day, but not on their ass - as in they always have plenty of work to do.

Suffix:

HERS (owned by a girl = it's hers)

Title:

When something withers, it fades away.

• you are very nearly there, just need to finish up now. – Shahriar Mahmud Sajid Sep 12 '18 at 8:50
• I was about to post exactly the same answer. I too have no idea what the infix is about – Kaspar Scherrer Sep 12 '18 at 8:50
• my best guess for infix is rot13('GU' fgnaqf sbe gnoyr urnqre (UGZY), juvpu fvgf nobir n gnoyr.) I doubt this is it though.. – Kaspar Scherrer Sep 12 '18 at 9:02
• Worth a try, have edited it in – AHKieran Sep 12 '18 at 9:05
• another infix idea: rot13('VG' crbcyr unir qrfx wbof fb gurl pna fvg nyy qnl, ohg abg ba gurve nff - nf va gurl nyjnlf unir cyragl bs jbex gb qb.) – Kaspar Scherrer Sep 12 '18 at 9:30

Are you

Whitney Houston?

My prefix is pretty clever and funny

WIT

My infix can sit, though not on his ass,

KNEE (can kneel, which is some sort of sitting)

My suffix is owned by someone,

YOU

though not by me, by a bonny lass.

?

Together:

Wit-knee you-????? is homophone with the start of the name of Whitney Houston, a bonny lass.

• an interesting answer, but not yet correct, you got a partial correct though. $+1$ for you – Shahriar Mahmud Sajid Sep 12 '18 at 8:43

This is my guess, inspired by @Cashbee's prefix.

Are you

withering?

Prefix

WIT

Infix

HER (because she can sit, but not on his ass. Though, "she" would fit better :( )

Suffix

RING (since rings are typically a female accessory. Or maybe, you in particular don't own a ring, while definitely some "bonny lass" does)