Tracks men inside sewers? (7)
I believe the answer is:
tailors
'sewers?' is the definition.
(I've seen this in another clue)
'tracks men inside' is the wordplay.
'tracks' becomes 'tails' ('tail' can be a synonym of 'track').
'men' becomes 'OR' (Other Ranks: non-commissioned soldiers).
'inside' means one lot of letters goes inside another.
'tails' enclosing 'or' is 'TAILORS'.
(Other definitions for tailors that I've seen before include "They make fitted clothes" , "Workers" , "Designers" .)