Crack -- mentally and physically? (2,2,6)
I believe the answer is:
go to pieces
'crack' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for go to pieces that I've seen before include "lose it?" , "Crack up" , "Break up entirely" , "Have a breakdown" , "Become so nervous as to be unable to function" .)