Bed has small price (4)
I believe the answer is:
cost
'price' is the definition.
(I know that price can be written as cost)
'bed has small' is the wordplay.
'bed' becomes 'cot' (cot is a type of bed).
'has' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cot' going around 's' is 'COST'.
(Other definitions for cost that I've seen before include "Charge" , "estimate price" , "Estimate the expense of doing" , "penalty" , "Fee" .)