People going underground badly restricted — they grumble (9)

I believe the answer is:

cavillers

'they grumble' is the definition.
(to cavil can mean to complain)

'people going underground badly restricted' is the wordplay.
'people going underground' becomes 'cavers'.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'restricted' is an insertion indicator.
'cavers' going around 'ill' is 'CAVILLERS'.

(Another definition for cavillers that I've seen is " They find fault".)

I've seen this clue in The Guardian.

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