Hair fastener for rugby player (4)
I believe the answer is:
lock
I believe this is a triple definition!
'hair' is the first definition.
(a lock of hair)
'fastener' is the second definition.
(lock is a kind of fastener)
'for rugby player' is the third definition.
(second row of a scrum)
(Other definitions for lock that I've seen before include "forward" , "Secure - position in rugby team" , "A little hair" , "temporary container for lighter, say" , "Fasten securely" .)