Support friend in law (7)
I believe the answer is:
legally
'in law' is the definition.
(I've seen this before)
'support friend' is the wordplay.
'support' becomes 'leg' (leg is a kind of support).
'friend' becomes 'ally' (I've seen this before).
'leg'+'ally'='LEGALLY'
(Other definitions for legally that I've seen before include "As regards the law" , "without offence" , "according to the national regulations" , "In a manner approved by law" , "Licitly" .)