Lower G-string pinching back of crotch - one is in pain (9)
I believe the answer is:
diphthong
'one is in pain' is the definition.
(the 'ai' in 'pain' is a diphthong)
'lower g-string pinching back of crotch' is the wordplay.
'lower' becomes 'dip' (dipping is a kind of lowering).
'g-string' becomes 'thong' (type of underwear).
'pinching' means one lot of letters goes inside another (taking inside).
'back of' suggests the final letters.
The final letter of 'crotch' is 'h'.
'dip'+'thong'='dipthong'
'dipthong' enclosing 'h' is 'DIPHTHONG'.
(Other definitions for diphthong that I've seen before include "Combined vowel" , "Two vowels pronounced as one syllable" , "Sound of two vowels in one syllable" , "the thing in middle of cruise" , "Two-vowel sound" .)