Collapse in small shapeless mass (5)
I believe the answer is:
slump
'collapse' is the definition.
(I know that slump is a more specific form of the action collapse)
'small shapeless mass' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'shapeless mass' becomes 'lump' (I've seen this before).
's'+'lump'='SLUMP'
'in' is the link.
(Other definitions for slump that I've seen before include "Fall heavily and limply" , "Economic depression" , "Marked decline" , "Sit badly" , "Commercial decline" .)