Burning a sack (5)

I believe the answer is:

afire

'burning' is the definition.
(thesaurus)

'a sack' is the wordplay.
'sack' becomes 'fire' ('fire' can be a synonym of 'sack').
'a'+'fire'='AFIRE'

(Other definitions for afire that I've seen before include "Burning" , "in flames" , "Blazing - excited" , "Alight" , "lit up" .)

I've seen this clue in The Telegraph.

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