Call engineer about rear of contraption (4)
I believe the answer is:
ring
'call' is the definition.
('ring' can be a synonym of 'call')
'engineer about rear of contraption' is the wordplay.
'engineer' becomes 'rig' (I've seen this before**).
'about' indicates putting letters inside.
'rear of' suggests the final letters.
The final letter of 'contraption' is 'n'.
'rig' placed around 'n' is 'RING'.
(Other definitions for ring that I've seen before include "Marriage token" , "Group operating illegally < call" , "sign of union?" , "Circle that's square for boxers" , "Make phone call" .)