The girl looks different having slimmed (7)
I believe the answer is:
lighter
'having slimmed' is the definition.
I can't judge whether this definition defines the answer.
'the girl looks different' is the wordplay.
'looks different' indicates anagramming the letters (I've seen 'looking different' mean this (I've seen this in other clues)).
'the'+'girl'='thegirl'
'thegirl' is an anagram of 'LIGHTER'.
Can you help me to learn more?
(Other definitions for lighter that I've seen before include "Barge (more likely to float?)" , "Ship to shore boat" , "Canal boat with a flat bottom" , "Tender, launch" , "Boat for heavy loads and fire igniter" .)