Standard railway guard! (5)
I believe the answer is:
parry
'guard' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'standard railway' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'railway' becomes 'ry' (abbreviation for railway).
'par'+'ry'='PARRY'
Can you help me to learn more?
(Other definitions for parry that I've seen before include "Block - deflect" , "Duck" , "Jerusalem's composer" , "evasive action" , "Ward off - English composer, d. 1918" .)