Formerly working with chief engineer (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'working with chief engineer' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'with' is a charade indicator (letters next to each other).
'chief engineer' becomes 'ce' (this could be a standard abbreviation which I don't know about).
'on'+'ce'='ONCE'
Can you help me to learn more?
(Other definitions for once that I've seen before include "In former times" , "On a single (or past) occasion" , "'... bitten, twice shy'" , "in bygone days" , "yesterday" .)