Union movement to stop sale (7)
I believe the answer is:
auction
'sale' is the definition.
(I've seen this before)
'union movement to stop' is the wordplay.
I cannot really understand how this works, but
'union' could be 'u' (common abbreviation as in 'Trades Union Congress') and 'u' is found within the answer.
'movement' could be 'action' and 'action' is located in the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for auction that I've seen before include "Public sale with lots" , "Lots are sold here!" , "Offer to bidders" , "event with lots involved" , "Selling" .)