Rex leaving gloss to fade away (6)
I believe the answer is:
vanish
'fade away' is the definition.
(thesaurus)
'rex leaving gloss' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'leaving' suggests deleting specific letters.
'gloss' becomes 'varnish' (I've seen this before).
'varnish' with 'r' removed is 'VANISH'.
'to' acts as a link.
(Other definitions for vanish that I've seen before include "Disappear completely" , "Disappear without trace" , "make go" , "Disappear suddenly" , "Go missing" .)