Single's in line for ecstasy (8)
I believe the answer is:
paradise
'ecstasy' is the definition.
(thesaurus)
'single's in line' is the wordplay.
'single's' becomes 'I's' (one).
'in' means one lot of letters goes inside another.
'line' becomes 'parade' (eg a parade of shops).
'is' inserted into 'parade' is 'PARADISE'.
'for' is the link.
(Other definitions for paradise that I've seen before include "Idyllic place" , "Elysian fields" , "Poem by 1" , "What was lost to the poet" , "blissful place?" .)