Mirror? One friend holds it (7)
I believe the answer is:
Here is my best explanation:
'mirror?' is the definition.
'one friend holds it' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'friend' becomes 'mate' (mate is a kind of friend**).
'holds' means one lot of letters goes inside another.
'mate' going around 'it' is 'mitate'.
(Other definitions for imitate that I've seen before include "Copy, ape", "Mimic or follow as a model", "'Ape, copy (7)'", "Simulate", "Do", "Copy (mannerisms)", "Take as a model", "Mock", "I tame it (anag)" and "Copy or mimic".)