Back must be erect (4)
I believe the answer is:
rear
This clue is a double definition.
'back' is the first definition.
('rear' can be a synonym of 'back')
'erect' is the second definition.
('rear' can be a synonym of 'erect')
'must be' is the link.
(Other definitions for rear that I've seen before include "Bring up - back end" , "Buck" , "Rise up at the back" , "Bring up (children)" , "Bring up - the tail" .)