Girl has pain in both sides (6)
I believe the answer is:
'girl' is the definition.
'pain in both sides' is the wordplay.
'pain' becomes 'ache' (ache is a kind of pain).
'in' indicates putting letters inside.
'both sides' becomes 'RL' (abbreviation for left and right).
'ache' placed inside 'rl' is 'RACHEL'.
'has' acts as a link.
(Other definitions for rachel that I've seen before include "Heal RC of the girl" , "Woman" , "Second wife of Jacob" , "Import illicitly" , "Female" .)