He gets in a rut, mounting pistols
I believe the answer is:
stag
'he gets in a rut' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'mounting pistols' is the wordplay.
'mounting' says the letters should be written in reverse (going up in a down clue).
'pistols' becomes 'gats' (gat is a kind of pistol).
'gats' written backwards gives 'STAG'.
Can you help me to learn more?
(Other definitions for stag that I've seen before include "Male-only" , "Hunted animal" , "Male deer; speculator" , "Stockmarket operator gambling on a quick profit" , "A fully-grown male deer" .)