Cot's terrible expense (4)
I believe the answer is:
cost
'expense' is the definition.
(I know that expense can be written as cost)
'cot's terrible' is the wordplay.
'terrible' indicates an anagram.
'cots' with letters rearranged gives 'COST'.
(Other definitions for cost that I've seen before include "Suffering" , "Outlay required to buy" , "Be priced at" , "estimate price" , "penalty" .)