Return of one pound two shillings for ecstasy (5)
I believe the answer is:
bliss
'for ecstasy' is the definition.
(I've seen this before)
'return of one pound two shillings' is the wordplay.
'two shillings' means two substitutions for 'shillings'.
'return of' shows that the letters should be reversed in order.
'one' becomes 'I' (Roman numeral).
'pound' becomes 'lb' (abbreviation for the unit of weight).
'shillings' becomes 's' (abbreviation e.g. Lsd - pounds, shillings, pence).
'shillings' becomes 's' (abbreviation e.g. Lsd - pounds, shillings, pence).
'i'+'lb'='ilb'
'ilb' written backwards gives 'bli'.
'bli'+'s'+'s'='BLISS'
(Other definitions for bliss that I've seen before include "Beatitude" , "Great happiness" , "One who scored" , "promise of afterlife" , "Highest happiness" .)