Laugh when her colt misbehaves (7)
I believe the answer is:
chortle
'laugh' is the definition.
(I know that chortle is a type of laugh)
'her colt misbehaves' is the wordplay.
'misbehaves' indicates an anagram (I've seen 'misbehaving' mean this).
'her'+'colt'='hercolt'
'hercolt' with letters rearranged gives 'CHORTLE'.
'when' is the link.
(Other definitions for chortle that I've seen before include "Noisy gleeful laugh" , "Guffaw" , "Chuckle gleefully" , "Joyous chuckle" , "Titter" .)