Firm with a spell in trouble has to fold (8)
I believe the answer is:
collapse
'to fold' is the definition.
(I know this)
'firm with a spell in trouble' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' is an insertion indicator.
'in trouble' indicates an anagram.
'spell' is an anagram of 'llpse'.
'a' placed into 'llpse' is 'llapse'.
'co'+'llapse'='COLLAPSE'
'has' acts as a link.
(Other definitions for collapse that I've seen before include "Abrupt failure of function" , "Faint and fall" , "Slump" , "Fall down suddenly like escallop" , "Go to ruin" .)