Pair with say back pain (6)
I believe the answer is:
twinge
'pain' is the definition.
(twinge is a kind of pain)
'pair with say back' is the wordplay.
'pair' becomes 'twin' ('twin' can be a synonym of 'pair'**).
'with' is a charade indicator (letters next to each other).
'say' becomes 'eg'.
'back' says the letters should be written in reverse.
'eg' in reverse letter order is 'ge'.
'twin'+'ge'='TWINGE'
(Other definitions for twinge that I've seen before include "Sudden darting pain" , "A small stab of pain" , "A small pang, of pain or concience" , "A slight stab of pain" , "Sudden stab of pain" .)