Rugby halfback is hard, unfriendly (11)
I believe the answer is:
standoffish
'unfriendly' is the definition.
('standoffish' can be a synonym of 'unfriendly')
'rugby halfback is hard' is the wordplay.
'rugby halfback' becomes 'standoff' (I've seen this before).
'hard' becomes 'h'.
'standoff'+'is'+'h'='STANDOFFISH'
(Other definitions for standoffish that I've seen before include "Reserved, aloof" , "Unfriendly" , "like a half of rugby?" , "Distant" , "Cold" .)