Looters stumble on the way up, drinking beer (9)

I believe the answer is:

pillagers

'looters' is the definition.
('pillager' can be a synonym of 'looter')

'stumble on the way up drinking beer' is the wordplay.
'stumble' becomes 'slip' ('slip' can be a synonym of 'stumble').
'on the way up' is a reversal indicator.
'drinking' is an insertion indicator.
'beer' becomes 'lager' (lager is a kind of beer).
'slip' reversed gives 'pils'.
'pils' enclosing 'lager' is 'PILLAGERS'.

(Another definition for pillagers that I've seen is " Looters".)

I've seen this clue in The Telegraph.

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