By that cheapest possible settee, cot collapses (4,5)
I believe the answer is:
ipso facto
'by that' is the definition.
(I've seen this before)
'cheapest possible settee cot collapses' is the wordplay.
'cheapest possible' becomes 'ip' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'settee' becomes 'sofa' (I've seen this before).
'collapses' indicates anagramming the letters.
'cot' with letters rearranged gives 'cto'.
'ip'+'sofa'+'cto'='IPSO-FACTO'
Can you help me to learn more?
(Other definitions for ipso facto that I've seen before include "For that precise reason" , "Self-evidently; thereby" , "By that very deed" , "By that very act (Lat.)" , "by the very nature of the case)" .)