Good man, say, joining soldiers in piece (7)
I believe the answer is:
segment
'piece' is the definition.
(segment is a kind of piece)
'good man say joining soldiers in' is the wordplay.
'good man' becomes 'st' (abbreviation for saint).
'say' becomes 'eg' (both can mean 'for example').
'joining' is an insertion indicator (inserted letters join the word).
'soldiers' becomes 'men' (men can mean a group of soldiers).
'in' means one lot of letters goes inside another.
'st' enclosing 'eg' is 'segt'.
'segt' placed around 'men' is 'SEGMENT'.
(Other definitions for segment that I've seen before include "Portion" , "Part, piece" , "Part of, eg, an orange" , "Gets men with part cut off" , "One of several parts of a whole" .)