The final answer is:
Alsatian
Getting there:
First, some answers to the clues around the grid:
Left:
Animal fibre; Wool
"Fight For You" singer; H.E.R
Mesopotamian goddess, daughter of Anu - Bau
Owned - Had
Potassium - K
Quarrel - Row
Roseanne? - Barr
Samos International Airport - IATA code SMI
Right:
15 in hexadecimal - F
Deprecated Microsoft web browser - I.E. (Internet Explorer)
Georgia (country) - GE (country code)
James Bond's boss - M
King, or Cheadle? - Don
Lotidae member - Ling
Period of darkness - Night
Thorium - Th
Top:
Boot - Wellington
Grandmother - Nana
Incisor - Fang
Redden - Flush
Rock band - Toto
Royal Navy 'soldier slang' - Pongo
Scottish girl - Lassie
White - Snowy
Bottom:
These are all anagrams of dog breeds:
ADD OWN FUNNEL - Newfoundland
FIRE WRITER OR EX - Wire Fox Terrier
INERT CARRIER - Cairn Terrier
LATIN ADAM - Dalmatian
LOOPED - Poodle
OCEAN PRICKLES - Cocker Spaniel
ORIOLE GULCH - Rough Collie
URBAN HOOD - Boarhound
Graphically:

Putting it all together
The top and bottom give us the names and breeds of (mostly) fictional dogs, with the left and right joining to give us the surnames of authors who wrote about them:
Flush - Cocker Spaniel from Flush: A Biography (partly fictional, but about a real dog, hence the "mostly" above)
Wool+F - Virginia Woolf
Fang - Boarhound from the Harry Potter series
Row+Ling - J.K Rowling
Pongo - Dalmatian from The Hundred and One Dalmations
SMI+Th - Dodie Smith
Nana - Newfoundland from Peter Pan
Barr+IE - J.M. Barrie
Toto - Cairn Terrier from The Wonderful Wizard of Oz
Bau+M - L. Frank Baum
Snowy - Wire Fox Terrier from The Adventures of Tintin
H.E.R+GE - Hergé (pen name of Georges Prosper Remi)
Wellington - Poodle from The Curious Incident of the Dog in the Night-Time
Had+Don - Mark Haddon
Lassie - Rough Collie from Lassie Come Home
K+Night - Eric Knight
Finally:
Taking a hint from the X in the puzzle title, and the one in the centre of the grid, let's join up the elements and see where the lines cross:

And reading off the circled letter in order gives us ALSATIAN