Dilute good cup of tea (5)
I believe the answer is:
thing
'cup of tea' is the definition.
(I've seen this before)
'dilute good' is the wordplay.
'dilute' becomes 'thin' (synonyms).
'good' becomes 'g' (abbreviation).
'thin'+'g'='THING'
(Other definitions for thing that I've seen before include "Obsession" , "Unspecified object" , "An unnamed object or item" , "Individual unit" , "Affair - matter" .)