Formerly working with civil engineer (4)
I believe the answer is:
once
'formerly' is the definition.
(synonyms)
'working with civil engineer' is the wordplay.
'working' becomes 'on'.
'with' says to put letters next to each other.
'civil engineer' becomes 'ce'.
'on'+'ce'='ONCE'
(Other definitions for once that I've seen before include "Some time back" , "Long ago" , "'... bitten, twice shy'" , "In days past" , "yesterday" .)