Literary assistant a guy uses in ... in error (10)
I believe the answer is:
amanuensis
'literary assistant' is the definition.
(I've seen this before)
'a guy uses in in error' is the wordplay.
'guy' becomes 'man' (guy can informally mean a man).
'in error' indicates an anagram.
'uses'+'in'='usesin'
'usesin' anagrammed gives 'uensis'.
'a'+'man'+'uensis'='AMANUENSIS'
(Other definitions for amanuensis that I've seen before include "PA" , "Secretary" , "One assisting writer" , "stenographer" , "dictator's collaborator, perhaps" .)