One may have set up firm sink (7)
I believe the answer is:
'sink' is the definition.
(founder can mean to fill with water and sink)
'one may have set up firm' is the wordplay.
I cannot quite see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for founder that I've seen before include "Capsize, being an originator" , "One who establishes an institution" , "One sets up firm" , "(Of ship) fill with water and sink" , "collapse" .)