The head gives old boy lines! (7)
I believe the answer is:
obverse
'the head' is the definition.
(thesaurus)
'old boy lines' is the wordplay.
'old boy' becomes 'ob'.
'lines' becomes 'verse' (I've seen this before).
'ob'+'verse'='OBVERSE'
'gives' acts as a link.
(Other definitions for obverse that I've seen before include "Side of a coin that shows heads" , "Verbose on the opposite side" , "Side of coin that bears head or principle design" , "Coin's head side" , "One side of a coin" .)