Reefer could be hip (5)
I believe the answer is:
joint
This clue is a double definition.
'reefer' is the first definition.
('joint' can be a synonym of 'reefer')
'be hip' is the second definition.
(thesaurus)
'could' is the link.
(Other definitions for joint that I've seen before include "Hip maybe" , "Point in the body where two bones meet" , "Cut of meat; mutual" , "Bone junction" , "Piece of meat for roasting" .)