Speed up in pop-metal (5)
I believe the answer is:
tempo
'speed' is the definition.
(I've seen this before)
'up in pop-metal' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'opmet' is hidden within 'pop metal'.
'opmet' back-to-front is 'TEMPO'.
(Other definitions for tempo that I've seen before include "Muscial time" , "Speed at which piece of music is played" , "Rate and rhythm in music" , "Musical time" , "Rhythm, metre" .)