Fan replacing marks with pence to lay down? (9)
I believe the answer is:
stipulate
'lay down?' is the definition.
(as in to stipulate or lay down a law)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for stipulate that I've seen before include "Insist on as part of agreement" , "Demand as part of agreement" , "Define, require" , "Specify as an essential part" , "Make a demand" .)