Paddy has an afterthought about these costs (6)
I believe the answer is:
prices
'these costs' is the definition.
('price' can be a synonym of 'cost')
'paddy has an afterthought about' is the wordplay.
'paddy has' becomes 'rice' (I can't justify this - if you can you should give a lot more credence to this answer).
'an afterthought' becomes 'ps' (as in a PS in a letter).
'about' is an insertion indicator.
'rice' placed inside 'ps' is 'PRICES'.
Can you help me to learn more?
(Other definitions for prices that I've seen before include "Costs" , "Determines the cost (of)" , "Monetary values" .)