The girl messing about in a boat (7)
I believe the answer is:
lighter
'a boat' is the definition.
(lighter is a kind of boat)
'the girl messing about' is the wordplay.
'messing about' indicates an anagram.
'the'+'girl'='thegirl'
'thegirl' is an anagram of 'LIGHTER'.
'in' is the link.
(Other definitions for lighter that I've seen before include "Supply boat" , "Vessel" , "Fire-raiser" , "less tonnage" , "Less weighty" .)