Old Peruvian slope is glowing (12)
I believe the answer is:
incandescent
'is glowing' is the definition.
(I've seen this before)
'old peruvian slope' is the wordplay.
'old peruvian' becomes 'incan' (I've seen this in another clue).
'slope' becomes 'descent' (descent is a kind of slope).
'incan'+'descent'='INCANDESCENT'
(Other definitions for incandescent that I've seen before include "frenzied ..." , "White-hot" , "Glowing with heat" , "extremely angry" , "Glowing brightly" .)