Groom's worthless horse consumed by quartet (8)
I believe the answer is:
titivate
'groom's' is the definition.
(thesaurus)
'worthless horse consumed by quartet' is the wordplay.
'worthless horse' becomes 'tit' (I can't justify this - if you can you should believe this answer much more).
'consumed' becomes 'ate' (to eat is to consume food).
'by' says to put letters next to each other.
'quartet' becomes 'IV' (Roman numerals for four).
'ate' after 'iv' is 'ivate'.
'tit'+'ivate'='TITIVATE'
Can you help me to learn more?
(Other definitions for titivate that I've seen before include "Informally smarten up, put finishing rouches to" , "Primp" , "Doll oneself up" , "Smarten (oneself) up" , "Smarten up, put finishing touches on" .)