Intimate change and say nothing (5,3)
I believe the answer is:
alter ego
'intimate' is the definition.
('alter ego' can be a synonym of 'intimate')
'change and say nothing' is the wordplay.
'change' becomes 'alter' ('alter' can be a synonym of 'change').
'and' means one lot of letters go next to another.
'say' becomes 'eg'.
'nothing' becomes 'o' (looks like zero - 0**).
'alter'+'eg'+'o'='ALTER EGO'
(Other definitions for alter ego that I've seen before include "Similar character" , "Eg, to earl? (Anag)" , "'One's alternative personality (5,3)'" , "A second personality or intimate friend" , "bosom buddy" .)