Girl's hair trouble (8)
I believe the answer is:
distress
'trouble' is the definition.
(distressing is a kind of troubling)
'girl's hair' is the wordplay.
'girl's' becomes 'dis'.
'hair' becomes 'tress' (tress is a kind of hair).
'dis'+'tress'='DISTRESS'
(Other definitions for distress that I've seen before include "Cause mental pain" , "Adversity" , "Extreme anxiety or pain" , "Affliction" , "Upset" .)