The girl is upset by the boat (7)
I believe the answer is:
lighter
'boat' is the definition.
(I know that lighter is a type of boat)
'the girl is upset by' is the wordplay.
'is upset by' is an anagram indicator.
'the'+'girl'='thegirl'
'thegirl' anagrammed gives 'LIGHTER'.
'the' is the link.
(Other definitions for lighter that I've seen before include "Canal boat with a flat bottom" , "Boat with a flat bottom for carrying heavy loads" , "more blonde" , "Boat for heavy loads and fire igniter" , "Flat-bottomed boat" .)