Product on sale just before Christmas maybe to become less successful? (7)
I believe the answer is:
decline
'product on sale' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'just before christmas maybe to become less successful?' is the wordplay.
I cannot really see how this works, but
'christmas' could be 'dec' (Dec is short for December, the Christmas period) and 'dec' is found within the answer.
'successful?' could be 'in' (similar in meaning) and 'in' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for decline that I've seen before include "failure" , "Refuse; decay" , "Say no or lose strength" , "Gradually go down" , "go through cases" .)