Jack, a friend to many (7)
I believe the answer is:
matelot
'jack' is the definition.
(thesaurus)
'a friend to many' is the wordplay.
'a friend' becomes 'mate' (mate is a kind of friend).
'to many' becomes 'lot' (I've seen this before).
'mate'+'lot'='MATELOT'
(Other definitions for matelot that I've seen before include "One sailor" , "A lot met (anag) - sailor" , "Sailor (slang from French)" , "One at sea" , "Rating" .)