Sewers unnecessary for the most part (7)
I believe the answer is:
needles
'sewers' is the definition.
(I've seen this before)
'unnecessary for the most part' is the wordplay.
'unnecessary' becomes 'needless' (similar in meaning).
'for the most part' means to remove the last letter.
'needless' with its last letter removed is 'NEEDLES'.
(Other definitions for needles that I've seen before include "Western point of the Isle of Wight" , "Pins and - -" , "Is annoying" , "Sewers; annoys" , "Winds up" .)