Pro might, abandoning civil engineer (3)
I believe the answer is:
for
'pro' is the definition.
(thesaurus)
'might abandoning civil engineer' is the wordplay.
'might' becomes 'force' (I've seen this before).
'abandoning' indicates named letters should be taken away.
'civil engineer' becomes 'ce'.
'force' with 'ce' removed is 'FOR'.
(Other definitions for for that I've seen before include "On behalf of" , "On the side of" , "Favouring" , "See 24" , "Approving" .)