About the G-string - it’s a pain! (6)
I believe the answer is:
twinge
'it's a pain' is the definition.
(twinge is a kind of pain)
'about the g-string' is the wordplay.
'about' indicates putting letters inside.
'string' becomes 'twine' (synonyms).
'g' put inside 'twine' is 'TWINGE'.
(Other definitions for twinge that I've seen before include "A small stab of pain" , "Brief sharp pain" , "A small pang, of pain or concience" , "Sharp localised pain" , "Spasm" .)