Collapse of square block (5)
I believe the answer is:
slump
'collapse' is the definition.
(slumping is a kind of collapsing)
'square block' is the wordplay.
'square' becomes 's' (this could be a standard abbreviation of which I'm unaware).
'block' becomes 'lump'.
's'+'lump'='SLUMP'
'of' is the link.
Can you help me to learn more?
(Other definitions for slump that I've seen before include "Commercial decline" , "Sudden fall as in share values" , "Sudden market drop" , "Sudden decline" , "Fall heavily and limply" .)