Ape? One friend cages it (7)
I believe the answer is:
imitate
'ape?' is the definition.
('imitate' can be a synonym of 'ape')
'one friend cages it' is the wordplay.
'one' becomes 'i' (Roman numeral).
'friend' becomes 'mate' (mate is a kind of friend).
'cages' means one lot of letters goes inside another.
'mate' placed around 'it' is 'mitate'.
'i'+'mitate'='IMITATE'
(Other definitions for imitate that I've seen before include "I ate Tim! (anag.)" , "'Ape, copy (7)'" , "Copy someone" , "Reproduce" , "Mimic or follow as a model" .)