Main bit on the side short of one sewer (10)
I believe the answer is:
seamstress
'sewer' is the definition.
(I've seen this before)
'main bit on the side short of one' is the wordplay.
'main' becomes 'sea' (synonyms).
'bit on the side' becomes 'mistress' (both can mean a secret lover).
'short of' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'mistress' with 'i' taken away is 'mstress'.
'sea'+'mstress'='SEAMSTRESS'
(Other definitions for seamstress that I've seen before include "Dressmaker" , "needlewoman" , "sewer" , "Woman who sews for a living" , "Midinette" .)