Initially easy puzzle becomes hell (6)
I believe the answer is:
erebus
'hell' is the definition.
(thesaurus)
'initially easy puzzle' is the wordplay.
'initially' indicates taking the first letters.
'puzzle' becomes 'rebus' (type of visual puzzle).
The first letter of 'easy' is 'e'.
'e'+'rebus'='EREBUS'
'becomes' is the link.
(Other definitions for erebus that I've seen before include "Greek god of darkness, son of Chaos" , "Be sure (anag) - Greek god" , "The abode of the dead" , "Mt. -, Antarctic volcano; be sure (anag.)" , "It's hellish dark" .)