The pin of a revolver? (4)
I believe the answer is:
axis
'the pin' is the definition.
(thesaurus)
'a revolver?' is the wordplay.
'revolver?' becomes 'xis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'xis'='AXIS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for axis that I've seen before include "On which a body spins" , "league" , "Imaginary line around which a body rotates" , "x or y line on a graph" , "Imaginary line through the centre of an object on which it spins" .)