Pains in rear from nasty infection for the duration of the game (8)
I believe the answer is:
softball
'pains in rear from nasty infection for' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'duration of the game' is the wordplay.
'duration of' becomes 'soft' (I can't explain this - if you can you should give a lot more credence to this answer).
'the game' becomes 'ball' (ball game is a kind of game).
'soft'+'ball'='SOFTBALL'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for softball that I've seen before include "played by young American" , "sport!"" , "Junior variant of baseball" , "game" .)