Cruel MP's unexpected collapse (7)
I believe the answer is:
crumple
'collapse' is the definition.
(crumpling is a kind of collapsing)
'cruel mp's unexpected' is the wordplay.
'unexpected' indicates anagramming the letters.
'cruel'+'mp'='cruelmp'
'cruelmp' with letters rearranged gives 'CRUMPLE'.
(Other definitions for crumple that I've seen before include "Pucker" , "Crease" , "Collapse, cave in" , "Go to pieces" , "Suddenly lose composure" .)