Key clue solved - time is sweet (6)
I believe the answer is:
dulcet
'is sweet' is the definition.
(synonyms)
'key clue solved time' is the wordplay.
'key' becomes 'd' (musical key).
'solved' indicates anagramming the letters (the anagram must be solved).
'time' becomes 't' (abbreviation).
'clue' is an anagram of 'ulce'.
'd'+'ulce'+'t'='DULCET'
(Other definitions for dulcet that I've seen before include "(Of sound) melodious" , "Of voice or tone, sweet-sounding" , "Mellifluous, pleasing to the ear" , "Soft and pleasant sounding" , "Agreeable and soothing to the ear" .)