Silly daughter at the back (4)
I believe the answer is:
'silly' is the definition.
(I've seen this in other clues)
'daughter at the back' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'at' means one lot of letters go next to another.
'the back' becomes 'aft' (at the back of a ship).
(Other definitions for daft that I've seen before include "'---- as a brush': dotty" , "Scatty" , "Half-witted" , "Idiotic" , "Bats!" .)