Reveal terrible pain is constant (11)
I believe the answer is:
everlasting
'is constant' is the definition.
(thesaurus)
'reveal terrible pain' is the wordplay.
'terrible' indicates an anagram.
'pain' becomes 'sting' (sting is a kind of pain**).
'reveal' is an anagram of 'everla'.
'everla'+'sting'='EVERLASTING'
(Other definitions for everlasting that I've seen before include "describes Wonka's gobstoppers" , "Timeless" , "Unending" , "Eternal" , "Unceasing" .)