Anticipate knocking back cider in exercise class (7)
I believe the answer is:
predict
'anticipate' is the definition.
(I've seen this before)
'knocking back cider in exercise class' is the wordplay.
'knocking back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'exercise class' becomes 'pt' (physical training).
'cider' reversed gives 'redic'.
'redic' going inside 'pt' is 'PREDICT'.
(Other definitions for predict that I've seen before include "Prophesy - portend" , "Estimate consequence of" , "Forecast" , "Guess (the future)" , "act as oracle" .)