What's left of engineers' team bringing in union (7)
I believe the answer is:
residue
'what's left' is the definition.
(residue means what's left behind)
'engineers' team bringing in union' is the wordplay.
'engineers'' becomes 'RE' (Royal Engineers).
'team' becomes 'side' (both can mean a sports team).
'bringing in' means one lot of letters goes inside another.
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'side' enclosing 'u' is 'sidue'.
're'+'sidue'='RESIDUE'
'of' is the link.
(Other definitions for residue that I've seen before include "Leftovers" , "Lees, dregs" , "Something left over" , "Something remaining" , "Whatever is left over" .)