Close friend breaking large toe (5,3)
I believe the answer is:
alter ego
'close friend' is the definition.
(alter ego can informally mean a close friend)
'breaking large toe' is the wordplay.
'breaking' indicates anagramming the letters.
'large'+'toe'='largetoe'
'largetoe' anagrammed gives 'ALTER EGO'.
(Other definitions for alter ego that I've seen before include "Is it Kent" , "Trusted friend" , "Change conceit for other self" , "Other self - large toe (anag)" , "'One's second self (5,3)'" .)