Set to work reassembling engine (3-6)
I believe the answer is:
two-stroke
'engine' is the definition.
(I've seen this before)
'set to work reassembling' is the wordplay.
'reassembling' indicates an anagram.
'set'+'to'+'work'='settowork'
'settowork' is an anagram of 'TWO-STROKE'.
(Other definitions for two-stroke that I've seen before include "Engine for eg motor mower" , "Small combustion engine" , "Type of internal-combustion engine" .)