A staggeringly insane sum for assistant (10)
I believe the answer is:
amanuensis
'for assistant' is the definition.
(amanuensis is a kind of assistant)
'a staggeringly insane sum' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for amanuensis that I've seen before include "stenographer" , "One assisting writer" , "secretarial post" , "dictator's collaborator, perhaps" , "Literary assistant - scribe" .)