Extremely deficient game is over (6)
I believe the answer is:
across
'over' is the definition.
(synonyms)
'extremely deficient game' is the wordplay.
'extremely deficient' indicates the central letters.
'game' becomes 'lacrosse' (lacrosse is a sport).
The centre of 'lacrosse' is 'ACROSS'.
'is' is the link.
(Other definitions for across that I've seen before include "what follows one here" , "To the opposite side" , "written like this?" , "Clue with solution not going in the direction of this one!" , "Clues like this" .)