Go over what appears in eruption (6)
I believe the answer is:
rehash
'go over' is the definition.
(as in rehashing an old argument)
'what appears in eruption' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'appears in' means one lot of letters goes inside another.
'eruption' becomes 'rash' (synonyms).
'eh' inserted inside 'rash' is 'REHASH'.
(Other definitions for rehash that I've seen before include "makeshift changes?" , "Rework old material" , "Makeover" , "Old material, slightly reworked and used again" , "Use old material in a new form" .)