Expense of keeping son in bed (4)
I believe the answer is:
cost
'expense' is the definition.
(cost is a kind of expense)
'keeping son in bed' is the wordplay.
'keeping' is an insertion indicator.
'son' becomes 's'.
'in bed' becomes 'cot' (cot is a kind of bed. I am not sure about the 'in' bit.).
's' going into 'cot' is 'COST'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for cost that I've seen before include "Amount to pay" , "Fee" , "Suffering" , "penalty" , "estimate price" .)