Some mighty keen Yorkshireman (4)
I believe the answer is:
tyke
'yorkshireman' is the definition.
(I've seen this before)
'some mighty keen' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'TYKE' can be found hidden inside 'mighty keen'.
(Other definitions for tyke that I've seen before include "Colloquially, a small naughty child" , "Naughty kid" , "A worthless dog" , "Cheeky kid" , "Mischievous child" .)