Her slot designed as gun holder (7)
I believe the answer is:
holster
'gun holder' is the definition.
(I've seen this before)
'her slot designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'her'+'slot'='herslot'
'herslot' anagrammed gives 'HOLSTER'.
'as' acts as a link.
(Other definitions for holster that I've seen before include "armrest" , "Sheath for a handgun" , "Pistol-holder" , "that carries a gun" , "put gun away" .)