On the door, he's full of gin - drunk! (5)
I believe the answer is:
hinge
'on the door he's full of gin drunk' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'he's full of gin drunk' is the wordplay.
'full of' is an insertion indicator.
'drunk' is an anagram indicator.
'gin' anagrammed gives 'ing'.
'he' going around 'ing' is 'HINGE'.
Can you help me to learn more?
(Other definitions for hinge that I've seen before include "On what a door swings" , "Key principle" , "Joint between door and frame" , "Circumstance upon which subsequent events depend; door facilitator" , "Pivotal point" .)