Standard beginning to slip in average cheese (8)
I believe the answer is:
parmesan
'cheese' is the definition.
(Parmesan is an example)
'standard beginning to slip in average' is the wordplay.
'standard' becomes 'par' (standard of play in golf).
'beginning to' indicates taking the first letters.
'in' is an insertion indicator.
'average' becomes 'mean' (type of average).
The first letter of 'slip' is 's'.
's' inserted into 'mean' is 'mesan'.
'par'+'mesan'='PARMESAN'
(Other definitions for parmesan that I've seen before include "Italian cheese for spearman" , "Hard dry cheese, often grated" , "Cheese for grating" , "Italian 8 across" , "that's grated" .)