Compelled civil engineer to split crossing (6)
I believe the answer is:
forced
'compelled' is the definition.
(forcing is a kind of compelling)
'civil engineer to split crossing' is the wordplay.
'civil engineer' becomes 'ce' ().
'to split' means one lot of letters goes inside another (some letters split a word into two).
'crossing' becomes 'ford' (synonyms).
'ce' put within 'ford' is 'FORCED'.
(Other definitions for forced that I've seen before include "Compelled, coerced" , "Not spontaneous" , "Like early rhubarb" , "Unnatural; false" , "Not natural" .)