Hat -- it goes on friend excellently (9)
I believe the answer is:
capitally
'excellently' is the definition.
(capital can mean excellent)
'hat it goes on friend' is the wordplay.
'hat' becomes 'cap' (I've seen this before).
'goes on' means one lot of letters go next to another.
'friend' becomes 'ally' (an ally is a friend or supporter).
'cap'+'it'+'ally'='CAPITALLY'
(Other definitions for capitally that I've seen before include "Admirably" , "in a fine manner" , "Very well" .)