Files a letter as an afterthought (5)
I believe the answer is:
rasps
'files' is the definition.
(I know this)
'a letter as an afterthought' is the wordplay.
'a letter' becomes 'r' ().
'an afterthought' becomes 'ps' (as in a PS in a letter).
'r'+'as'+'ps'='RASPS'
(Other definitions for rasps that I've seen before include "Jars" , "fruit briefly" , "Scrapes" , "speaks hoarsely" , "Coarse files" .)