Mock ultimately decadent leisure (5)
I believe the answer is:
tease
'mock' is the definition.
(teasing is a kind of mocking)
'ultimately decadent leisure' is the wordplay.
'ultimately' suggests the final letters.
'leisure' becomes 'ease' (I've seen this before).
The final letter of 'decadent' is 't'.
't'+'ease'='TEASE'
(Other definitions for tease that I've seen before include "Use a comb" , "Poke fun at" , "Annoy good-naturedly" , "Torment lightly" , "Vex" .)