Where accessory may be left in the gap that's up the wall (6)
I believe the answer is:
hatpeg
'where accessory may be left' is the definition.
(hats are accessories)
'the gap that's up the wall' is the wordplay.
'that's up the wall' is an anagram indicator (up the wall can mean angry or mad).
'the'+'gap'='thegap'
'thegap' is an anagram of 'HATPEG'.
'in' is the link.
(Other definitions for hatpeg that I've seen before include "Cloakroom pin" , "where Kentucky Derby's held?" , "fixture for bowler?" , "Where to hang cap, etc" .)