One guarding striker receiving back pay (9)
I believe the answer is:
beefeater
'one guarding' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'striker receiving back pay' is the wordplay.
'striker' becomes 'beater' (I can't explain this - if you can you should believe this answer much more).
'receiving' indicates putting letters inside.
'back' shows that the letters should be reversed in order.
'pay' becomes 'fee'.
'fee' in reverse letter order is 'eef'.
'beater' going around 'eef' is 'BEEFEATER'.
Can you help me to learn more?
(Other definitions for beefeater that I've seen before include "no vegetarian?" , "Tower of London guard" , "One might guard" , "Yeoman of the Guard; carnivore" , "Bodyguard of the British monarch" .)