Sibling, say, back in pain (6)
I believe the answer is:
twinge
'pain' is the definition.
(twinge is a kind of pain)
'sibling say back' is the wordplay.
'sibling' becomes 'twin' (twin is a kind of sibling).
'say' becomes 'eg' (both can mean 'for example').
'back' shows that the letters should be reversed in order.
'eg' in reverse letter order is 'ge'.
'twin'+'ge'='TWINGE'
'in' is the link.
(Other definitions for twinge that I've seen before include "Slight pain (of conscience?)" , "Small, sharp pain" , "Qualm" , "Slight pang, maybe of conscience" , "A small pang, of pain or concience" .)