Impromptu strike after rotten article appears (3,3,4)
I believe the answer is:
off the cuff
'impromptu' is the definition.
(I've seen this before)
'strike after rotten article appears' is the wordplay.
'strike' becomes 'cuff' (I've seen this before).
'after' says to put letters next to each other.
'rotten' becomes 'off' (I've seen this before).
'article' becomes 'the' (the definite article in English).
'appears' means one lot of letters go next to another (some letters appear after others).
'off'+'the'='offthe'
'cuff' put after 'offthe' is 'OFF-THE-CUFF'.
(Other definitions for off the cuff that I've seen before include "Improvised - extempore" , "Impromptu" , "Unofficially - improvised" , "Where one may lose shirt button" , "From where you could lose a button" .)