Standard piece of rock salmon (4)
I believe the answer is:
parr
'salmon' is the definition.
(I've seen this before)
'standard piece of rock' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'piece of' indicates taking the first letters.
The initial letter of 'rock' is 'r'.
'par'+'r'='PARR'
(Other definitions for parr that I've seen before include "Wife surviving" , "Young salmon - wife of Henry VIII" , "survivor of the decapitator?" , "Young salmon or trout" , "Catherine --, last wife of Henry VIII" .)