Tell a friend off the record (10)
I believe the answer is:
informally
'off the record' is the definition.
(I've seen this before)
'tell a friend' is the wordplay.
'tell' becomes 'in form' (I've seen this before).
'a friend' becomes 'ally' (an ally is a friend or supporter).
'inform'+'ally'='INFORMALLY'
(Other definitions for informally that I've seen before include "off the record" , "in relaxed fashion" , "in relaxed manner" .)