Small crew for engine driver (5)
I believe the answer is:
steam
'for engine driver' is the definition.
(a steam engine is powered by steam)
'small crew' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'crew' becomes 'team' (both can mean a group of colleagues).
's'+'team'='STEAM'
(Other definitions for steam that I've seen before include "enthusiasm" , "A lot of hot air" , "Sort of water" , "Mates (anag)" , "Water at boiling temperature diffused in the atmosphere" .)