Spectator's running after large rugby player, having lost his head (6-2)

I believe the answer is:

looker-on

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'large rugby player having lost his head' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'spectator's running after' is the wordplay.
'spectator' becomes 'looker' (I've seen this in another clue).
'running' becomes 'on' (eg when a machine is running it is on).
'after' says to put letters next to each other.
'looker'+'on'='LOOKER-ON'

Can you help me to learn more?

(Another definition for looker-on that I've seen is " A (mere) spectator".)

I've seen this clue in The Independent.

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