Daughter gets terribly fat and stupid (4)
I believe the answer is:
daft
'stupid' is the definition.
(I've seen this before)
'daughter gets terribly fat' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'gets' is an insertion indicator.
'terribly' indicates an anagram.
'fat' is an anagram of 'aft'.
'd' inserted inside 'aft' is 'DAFT'.
'and' is the link.
(Other definitions for daft that I've seen before include "'---- as a brush': dotty" , "Informally silly or crazy" , "Idiotic" , "Informally silly or foolish" , "Potty" .)