Firm with large mistake in ruin (8)
I believe the answer is:
collapse
'ruin' is the definition.
(I've seen this before)
'firm with large mistake' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' says to put letters next to each other.
'large' becomes 'l'.
'mistake' becomes 'lapse' (lapse is a kind of mistake**).
'co'+'l'+'lapse'='COLLAPSE'
'in' acts as a link.
(Other definitions for collapse that I've seen before include "Fall down, building or business" , "Abrupt failure of function" , "Break-down or complete failure" , "Fall or disintergrate" , "Fall down suddenly like escallop" .)