Decline of following after one season (4,3)
I believe the answer is:
fall off
'decline' is the definition.
(I know that drop is the is a more specific form of the action of fall off.)
'of following after one season' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'after' means one lot of letters go next to another.
'one season' becomes 'fall' (alternative name for autumn).
'of'+'f'='off'
'off' put after 'fall' is 'FALL-OFF'.
(Other definitions for fall off that I've seen before include "Deterioration" , "deteriorate" , "How unsuccessful tightrope walker might" , "Decline" , "Decrease" .)