Go off, alternatively, to get an engine part (5)
I believe the answer is:
rotor
'an engine part' is the definition.
(I've seen this before)
'go off alternatively' is the wordplay.
'go off' becomes 'rot' (I've seen this before).
'alternatively' becomes 'or'.
'rot'+'or'='ROTOR'
'to get' is the link.
(Other definitions for rotor that I've seen before include "Chopper blade" , "Eddy" , "Part of, eg, a motor" , "Turner" , "Turning blade" .)