Grating, 50 per cent heavy metal, released smoke (7)
I believe the answer is:
irksome
'grating' is the definition.
(I've seen this in another clue)
'50 per cent heavy metal released smoke' is the wordplay.
'50 per cent' means to take half the letters.
'heavy metal' becomes 'iron' (I've seen this before).
'released' indicates an anagram (letters released from their original order).
'iron' cut in half is 'ir'.
'smoke' with letters rearranged gives 'ksome'.
'ir'+'ksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "'Annoying, irritating or boring (7)'" , "Smoker I find annoying" , "Bothersome, annoying" , "bugging" , "Tediously annoying" .)