A mash that is sheer ecstasy (5)
I believe the answer is:
puree
'a mash' is the definition.
(I've seen this before)
'sheer ecstasy' is the wordplay.
'sheer' becomes 'pure' (synonyms).
'ecstasy' becomes 'e' (informal term for the drug).
'pure'+'e'='PUREE'
'that is' acts as a link.
(Other definitions for puree that I've seen before include "process" , "Fruit pulp" , "Liquidised food" , "Vegetable pulp" , "Sieved food reduced to pulp" .)