Supply insane sum to retain a literary assistant (10)
I believe the answer is:
amanuensis
'literary assistant' is the definition.
(I've seen this before)
'supply insane sum to retain a' is the wordplay.
'supply' is an anagram indicator ('supply' is an adverb here, so in a 'supple' way).
'to retain' means one lot of letters goes inside another.
'insane'+'sum'='insanesum'
'insanesum' anagrammed gives 'amnuensis'.
'amnuensis' placed around 'a' is 'AMANUENSIS'.
(Other definitions for amanuensis that I've seen before include "One assisting writer" , "secretarial post" , "stenographer" , "PA" , "dictator's collaborator, perhaps" .)