Standard source of salt found in average cheese (8)
I believe the answer is:
parmesan
'cheese' is the definition.
(Parmesan is an example)
'standard source of salt found in average' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'source of' suggests taking the first letters.
'found in' means one lot of letters goes inside another.
'average' becomes 'mean' (type of average).
The initial letter of 'salt' is 's'.
's' placed into 'mean' is 'mesan'.
'par'+'mesan'='PARMESAN'
(Other definitions for parmesan that I've seen before include "Hard dry cheese, often grated" , "that's grated" , "Hard dry Italian cheese" , "Italian cheese for spearman" , "Italian cheese, often grated" .)