Engine’s time in dock (5)
I believe the answer is:
motor
'engine's' is the definition.
(I've seen this in another clue)
'time in dock' is the wordplay.
'time' becomes 't' (abbreviation).
'in' means one lot of letters goes inside another.
'dock' becomes 'moor' (I've seen this in other clues).
't' going within 'moor' is 'MOTOR'.
(Other definitions for motor that I've seen before include "Drive" , "Make good progress" , "Car (informal)" , "Move fast" , "Wheels" .)