Even heavy metal is smoother (4,4)
I believe the answer is:
flat iron
'is smoother' is the definition.
(something which 'smooths')
'even heavy metal' is the wordplay.
'even' becomes 'flat' ('flat' can be similar in meaning to 'even').
'heavy metal' becomes 'iron' (I've seen this before).
'flat'+'iron'='FLAT-IRON'
(Other definitions for flat iron that I've seen before include "Appliance once used for pressing clothes" , "Old domestic implement" , "Pressing implement" .)