Writer’s aim to hold loon, say, back (4,5)
I believe the answer is:
gore vidal
'writer's' is the definition.
(US writer and intellectual)
'aim to hold loon say back' is the wordplay.
'aim' becomes 'goal' (I've seen this before).
'to hold' indicates putting letters inside.
'loon say' becomes 'diver' (loon is a diving bird).
'back' says the letters should be written in reverse.
'diver' written backwards gives 'revid'.
'goal' going around 'revid' is 'GORE VIDAL'.
(Other definitions for gore vidal that I've seen before include "he writes" , "American novelist and playwright, b. 1925" .)