Puts one’s foot down firmly (6)
I believe the answer is:
stamps
'puts one's foot down firmly' is the definition.
I can't tell whether this definition defines the answer.
'foot down firmly' is the wordplay.
'foot' becomes 'stamp' (stamping is a kind of footing).
'down firmly' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'stamp'+'s'='STAMPS'
Can you help me to learn more?
(Other definitions for stamps that I've seen before include "Items of interest to a philatelist" , "Brings one's foot down heavily on the envelope" , "Cuts out with a die" , "Postal tokens" , "Gummed receipts of postal fees" .)