Male is mounted carrying expensive weapon (7)
I believe the answer is:
sidearm
'weapon' is the definition.
(I've seen this before)
'male is mounted carrying expensive' is the wordplay.
'male' becomes 'm' (common abbreviation).
'mounted' is a reversal indicator.
'carrying' indicates putting letters inside.
'expensive' becomes 'dear' (I've seen this before).
'm'+'is'='mis'
'mis' written backwards gives 'sim'.
'sim' enclosing 'dear' is 'SIDEARM'.
(Other definitions for sidearm that I've seen before include "Officer may carry this" , "weapon" , "One may be fired" , "Piece" .)