My prefix may well be you, and is followed by u
My suffix is an obsolete exercise
My infix is better when hard
My whole is often done in a line
Is it MANUFACTURE
Prefix MAN and U
Suffix not sure about this ACTURE?
Infix FACT, in the hard facts.
When goods are manufactured they are done in a line
$\begingroup$ Well. That was quick. What gave it away? $\endgroup$ Sep 17, 2020 at 18:19
$\begingroup$ The first clue was a dead give away. $\endgroup$ Sep 17, 2020 at 18:21
1$\begingroup$ Ah, well. Tried not to make it too hard. Wound up making it too easy. Suffix is not the one I was thinking, but is excellent nonetheless. Try smaller. $\endgroup$ Sep 17, 2020 at 18:22