A night in with back occasionally massaged? That'll never happen! (5,5)
I believe the answer is:
think again
'that'll never happen' is the definition.
('think again' can be used to express disbelief)
'a night in with back occasionally massaged?' is the wordplay.
'with' means one lot of letters go next to another.
'occasionally' indicates alternate letters.
'massaged?' indicates an anagram (I've seen 'massage' mean this).
The alternating letters of 'back' are 'ak'.
'a'+'night'+'in'+'ak'='anightinak'
'anightinak' anagrammed gives 'THINK AGAIN'.
(Other definitions for think again that I've seen before include "Reconsider" , "have another brood?" .)