Copy one thing in friend (7)
I believe the answer is:
imitate
'copy' is the definition.
(I know that imitate is a type of copy)
'one thing in friend' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thing' becomes 'it'.
'in' means one lot of letters goes inside another.
'friend' becomes 'mate' (mate is a kind of friend).
'it' going into 'mate' is 'mitate'.
'i'+'mitate'='IMITATE'
(Other definitions for imitate that I've seen before include "Mimic or follow as a model" , "Follow suit" , "Parrot" , "Reproduce" , "Take as a model" .)