Mother, awfully sad at heart, is absolutely crazy (3,2,1,6)
I believe the answer is:
mad as a hatter
'absolutely crazy' is the definition.
'mother awfully sad at heart' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'awfully' is an anagram indicator.
'sad'+'at'+'heart'='sadatheart'
'sadatheart' with letters rearranged gives 'dasahatter'.
'ma'+'dasahatter'='MAD AS A HATTER'
'is' is the link.
(Other definitions for mad as a hatter that I've seen before include "Crazy" , "Completely daft" , "Totally daft" , "Sun-orbiting body" , "Completely insane (informal)" .)