Union spent to bring in a civil engineer (8)
I believe the answer is:
alliance
'union' is the definition.
(alliance is a kind of union)
'spent to bring in a civil engineer' is the wordplay.
'spent' becomes 'allin' (I've seen this before).
'to bring in' means one lot of letters goes inside another.
'civil engineer' becomes 'ce'.
'allin' going around 'a' is 'allian'.
'allian'+'ce'='ALLIANCE'
(Other definitions for alliance that I've seen before include "Federation" , "Bond" , "Union for mutual benefit" , "Confederation" , "League - treaty" .)