Insects sat on by a girl (5)
I believe the answer is:
alice
'girl' is the definition.
'insects sat on by a' is the wordplay.
'insects' becomes 'lice' (louse is a kind of insect).
'sat on by' says to put letters next to each other.
'lice' after 'a' is 'ALICE'.
(Other definitions for alice that I've seen before include "Wondergirl" , "A Town Like - - (novel/film)" , "Wonderland girl's name" , "Lewis Carroll character" , "Miss Liddell" .)