Dive in - terribly delightful (6)
I believe the answer is:
divine
'delightful' is the definition.
(I've seen this before)
'dive in terribly' is the wordplay.
'terribly' indicates an anagram.
'dive'+'in'='divein'
'divein' is an anagram of 'DIVINE'.
(Other definitions for divine that I've seen before include "clergyman" , "Theologian" , "To do with deity" , "Godly or celestial" , "Godlike or extremely beautiful" .)