Gain from impetus when energy is injected (6)
I believe the answer is:
derive
'gain from impetus' is the definition.
'derive' can be an answer for 'gain' (thesaurus). I am not sure about the 'from impetus' bit.
'when energy is injected' is the wordplay.
'when' becomes 'e' (I've seen this in other clues).
'energy' becomes 'drive' (I've seen this before).
'is injected' is an insertion indicator.
'e' placed within 'drive' is 'DERIVE'.
(Other definitions for derive that I've seen before include "Originate (from something)" , "Get or trace from a source" , "Originate, come (from)" , "Issue" , "Infer; originate (from)" .)