You caught cat snaring mice behind mounted drains (4,2)
I believe the answer is:
uses up
'drains' is the definition.
(using up is a kind of draining)
'you caught cat snaring mice behind mounted' is the wordplay.
'caught' shows a homophone (sound like) (heard).
'cat' becomes 'puss' (puss is a kind of cat).
'snaring' is an insertion indicator.
'behind' says to take the final letters.
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
The last letter of 'mice' is 'e'.
'you' sounds like 'u'.
'puss' going around 'e' is 'puses'.
'puses' backwards is 'sesup'.
'u'+'sesup'='USES UP'
(Other definitions for uses up that I've seen before include "Exhausts" , "drains" , "Consumes" , "'Depletes (4,2)'" .)