Daughter gets terribly fat and foolish (4)
I believe the answer is:
daft
'foolish' is the definition.
(I've seen this before)
'daughter gets terribly fat' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'gets' means one lot of letters goes inside another.
'terribly' is an anagram indicator.
'fat' with letters rearranged gives 'aft'.
'd' placed inside 'aft' is 'DAFT'.
'and' acts as a link.
(Other definitions for daft that I've seen before include "Informally silly or foolish" , "'Silly, foolish (4)'" , "Bats!" , "Idiotic" , "'---- as a brush': dotty" .)