In back of building he rolled joint (5)
I believe the answer is:
hinge
'joint' is the definition.
(I've seen this before)
'in back of building he rolled' is the wordplay.
'back of' indicates one should take the final letters.
'rolled' indicates an anagram.
The final letter of 'building' is 'g'.
'in'+'g'+'he'='inghe'
'inghe' anagrammed gives 'HINGE'.
(Other definitions for hinge that I've seen before include "Door pivot" , "Pivotal point" , "Key principle" , "Joint between door and frame" , "Joint on which things turn" .)