A red set must be converted, sweetheart (7)
I believe the answer is:
dearest
'sweetheart' is the definition.
(I've seen this in another clue)
'a red set must be converted' is the wordplay.
'must be converted' indicates an anagram.
'a'+'red'+'set'='aredset'
'aredset' anagrammed gives 'DEAREST'.
(Other definitions for dearest that I've seen before include "Loved one" , "Darling" , "The beloved one is most costly" , "Most highly valued or most expensive" , "Most loved or expensive" .)