Duck duties requiring a keen enthusiasm to begin with (5)
I believe the answer is:
drake
'duck' is the definition.
(drake is a male duck)
'duties requiring a keen enthusiasm to begin with' is the wordplay.
'duties' becomes 'dra' (I can't explain this - if you can you should give a lot more credence to this answer).
'requiring' means one lot of letters go next to another.
'to begin with' suggests taking the first letters.
The initial letters of 'keen enthusiasm' is 'ke'.
'dra'+'ke'='DRAKE'
Can you help me to learn more?
(Other definitions for drake that I've seen before include "Might he have bowled" , "Nick" , "Male bird" , "it's used to fish" , "-'s Drum (Newbolt)" .)