Engine’s temperature fell all round (5)
I believe the answer is:
motor
'engine's' is the definition.
(I've seen this before)
'temperature fell all round' is the wordplay.
'temperature' becomes 't' (abbreviation).
'fell' becomes 'moor' (I've seen this before).
'all round' means one lot of letters goes inside another.
't' placed within 'moor' is 'MOTOR'.
(Other definitions for motor that I've seen before include "Machine that converts energy into motion" , "Driver" , "go quickly" , "Car; powered machine" , "Car (colloq,)" .)