Go with a f-firm? (9)
I believe the answer is:
accompany
'go with' is the definition.
(I've seen this before)
'a f-firm?' is the wordplay.
'f-firm?' becomes 'ccompany' ('company' said with a stutter like 'firm' was; I've seen this before).
'a'+'ccompany'='ACCOMPANY'
(Other definitions for accompany that I've seen before include "Travel along with" , "Go with as a companion" , "Unbelief" , "Escort" .)