Working part, or a stubborn fellow out to do wrong (6)
I believe the answer is:
module
'working part or a stubborn fellow' is the definition.
I can't tell whether this defines the answer.
'out to do wrong' is the wordplay.
'out' means one lot of letters goes inside another.
'to' becomes 'mule' (I am not sure about this - if you are sure you should believe this answer much more).
'wrong' is an anagram indicator.
'do' anagrammed gives 'od'.
'mule' placed around 'od' is 'MODULE'.
Can you help me to learn more?
(Other definitions for module that I've seen before include "Standardised part in furniture - or spacecraft" , "Detachable spacecraft unit" , "Element of course etc." , "Complete part (of spacecraft or course)" , "Detachable sub-unit" .)