Frivolous daughter near back of boat (4)
I believe the answer is:
daft
'frivolous' is the definition.
(both can mean silly)
'daughter near back of boat' is the wordplay.
'daughter' becomes 'd'.
'near back of boat' becomes 'aft'.
'd'+'aft'='DAFT'
(Other definitions for daft that I've seen before include "Informally silly or foolish" , "like a brush?" , "Scatty" , "'---- as a brush': dotty" , "Informally crazy as a brush" .)