Disheartened Ant or Dec changing style (3,4)
I believe the answer is:
art deco
'style' is the definition.
(decorative style)
'disheartened ant or dec changing' is the wordplay.
'disheartened' suggests removing the centre (the middle or heart is removed).
'changing' indicates an anagram.
'ant' with its middle removed is 'at'.
'at'+'or'+'dec'='atordec'
'atordec' anagrammed gives 'ART DECO'.
(Other definitions for art deco that I've seen before include "Redcoat (anag) -- inter-war design style" , "Visual design style - redcoat (anag)" , "Highly geometric style" , "pre-war style" , "Early C 20 style" .)