Ape -- one fellow captures it (7)
I believe the answer is:
imitate
'ape' is the definition.
('imitate' can be a synonym of 'ape')
'one fellow captures it' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fellow' becomes 'mate' (I've seen this before**).
'captures' indicates putting letters inside.
'mate' placed around 'it' is 'mitate'.
'i'+'mitate'='IMITATE'
(Other definitions for imitate that I've seen before include "Copy, simulate" , "Take as a model" , "Copy someone" , "'Ape, copy (7)'" , "I tame it (anag)" .)