Understood old men fall apart (2,2,6)
I believe the answer is:
go to pieces
'fall apart' is the definition.
('go to pieces' can be a synonym of 'fall apart')
'understood old men' is the wordplay.
'understood' becomes 'got' (getting is a kind of understanding).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'men' becomes 'pieces' ('piece' can be a synonym of 'man'**).
'got'+'o'+'pieces'='GO TO PIECES'
(Other definitions for go to pieces that I've seen before include "Have a breakdown" , "panic" , "Become so nervous as to be unable to function" , "Crack up" , "Break up entirely" .)