Fancy daughter -- mum intervenes furiously (4,3)
I believe the answer is:
like mad
'furiously' is the definition.
('like mad' can mean furiously or intensely)
'fancy daughter mum intervenes' is the wordplay.
'fancy' becomes 'like' (both can mean to find someone attractive).
'daughter' becomes 'd' (genealogical abbreviation).
'mum' becomes 'ma' (both can mean mother).
'intervenes' means one lot of letters goes inside another.
'like'+'d'='liked'
'liked' going around 'ma' is 'LIKE MAD'.
(Other definitions for like mad that I've seen before include "Very quickly" , "With great intensity, energy, or enthusiasm" , "wildly" , "Energetically" , "Furiously" .)