Not starting imagine unravelling puzzle (6)
I believe the answer is:
enigma
'puzzle' is the definition.
(thesaurus)
'not starting imagine unravelling' is the wordplay.
'not starting' means to remove the first letter.
'unravelling' is an anagram indicator.
'imagine' with its first letter taken away is 'magine'.
'magine' with letters rearranged gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "'Mystery, riddle (6)'" , "Difficult problem" , "'Puzzle, riddle (6)'" , "German military cipher machine" , "Mystery U puzzle" .)