Fighting in Metro regularly upset underground network (6)

I believe the answer is:

warren

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'underground network' is the definition.
'warren' can be an answer for 'network' (I have seen 'Network of burrows ' mean 'warren' so perhaps 'network' could also mean 'warren'). I'm not sure about the 'underground' bit.

'fighting in metro regularly upset' is the wordplay.
'fighting' becomes 'war'.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'upset' is a reversal indicator.
'in'+'metro'='inmetro'
The alternating letters of 'inmetro' are 'ner'.
'ner' back-to-front is 'ren'.
'war'+'ren'='WARREN'

Can you help me to learn more?

(Other definitions for warren that I've seen before include "Rabbit's burrow" , "Series of rabbit burrows" , "Colony of rabbits" , "Series of connected underground tunnels" , "Labyrinth of burrows" .)

I've seen this clue in The Independent.

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