Laugh at the vehicle Giles is leaving on (6)
I believe the answer is:
giggle
'laugh' is the definition.
(giggling is a kind of laughing)
'the vehicle giles is leaving on' is the wordplay.
'the vehicle' becomes 'gig' (type of horse-drawn carriage).
'leaving' indicates named letters should be taken away.
'on' means one lot of letters go next to another.
'giles' with 'is' taken away is 'gle'.
'gig'+'gle'='GIGGLE'
'at' is the link.
(Other definitions for giggle that I've seen before include "Laugh foolishly" , "Nervous or silly laughter" , "Laugh in a silly manner" , "Laugh nervously" , "Foolish or nervous laugh" .)