Reveal awful pain is constant (11)
I believe the answer is:
everlasting
'is constant' is the definition.
(thesaurus)
'reveal awful pain' is the wordplay.
'awful' indicates anagramming the letters.
'pain' becomes 'sting' (sting is a kind of pain).
'reveal' with letters rearranged gives 'everla'.
'everla'+'sting'='EVERLASTING'
(Other definitions for everlasting that I've seen before include "Undying" , "permanent" , "Eternal" , "description of the rest Romeo set up here?" , "Unceasing" .)