Sales opportunity to the French heading for Calais - I will interrupt a great many (7)
I believe the answer is:
auction
'sales opportunity' is the definition.
'to the french heading for calais i will interrupt a great many' is the wordplay.
'to the french' becomes 'au' ().
'heading for' indicates taking the first letters.
'will interrupt' is an insertion indicator.
'a great many' becomes 'ton' (as in a ton of something).
The first letter of 'calais' is 'c'.
'i' placed inside 'ton' is 'tion'.
'au'+'c'+'tion'='AUCTION'
(Other definitions for auction that I've seen before include "Caution about method of sale" , "Occasion of lots under the gavel" , "Lots are sold here!" , "Selling" , "Competitive sale" .)