Attractive girl - trim, that is (5)
I believe the answer is:
cutie
'attractive girl' is the definition.
(I've seen this before)
'trim that is' is the wordplay.
'trim' becomes 'cut' ('cut' can be a synonym of 'trim').
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'cut'+'ie'='CUTIE'
(Other definitions for cutie that I've seen before include "Attractive person or animal" , "Attractive girl" , "an attractive thing" , "Endearing person" , "something pleasing" .)