Beastly work for which a kind of engine is required (6)
I believe the answer is:
donkey
'for which a kind of engine is required' is the definition.
The definition suggests a singular noun which matches the answer.
'beastly work' is the wordplay.
I cannot quite see how this works, but
'work' could be 'do' (synonyms) and 'do' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for donkey that I've seen before include "Domesticated ass" , "Domesticated animal" , "Burro" , "Braying quadruped" , "See 21" .)