Keen old fellow coming round (4)
I believe the answer is:
moan
'keen' is the definition.
(I've seen this before)
'old fellow coming round' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fellow' becomes 'man' (I've seen this before**).
'coming round' indicates putting letters inside.
'o' going inside 'man' is 'MOAN'.
(Other definitions for moan that I've seen before include "Bewail" , "Complain; mournful sound" , "Complain, groan" , "Complain; rhymes with money lent" , "Beef" .)