Upstart involved in the action sale (7)
I believe the answer is:
auction
'sale' is the definition.
(I've seen this before)
'upstart involved in the action' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'involved in' indicates putting letters inside.
'u' placed within 'action' is 'AUCTION'.
(Other definitions for auction that I've seen before include "Where things are sold to the highest bidder" , "Occasion of lots under the gavel" , "Competitive sale" , "Caution about method of sale" , "Public sale with lots" .)