Caller rabbits on about purchasing a whole keg (6)

I believe the answer is:

barrel

I'm a little stuck... Click here to teach me more about this clue!

'a whole keg' is the definition.
'barrel' can be an answer for 'keg' (I've seen this in another clue). I am not sure about the 'a whole' bit.

'caller rabbits on about purchasing' is the wordplay.
'purchasing' says the answer is hidden in the clue.
'about' says the letters should be written backwards.
'lerrab' can be found hidden inside 'caller rabbits on'.
'lerrab' back-to-front is 'BARREL'.

Can you help me to learn more?

(Other definitions for barrel that I've seen before include "''Roll out the ..., let's have a ... of fun'' (WWII song)" , "Cask, drum" , "Cylindrical container for liquids" , "needs some oil" , "Round wooden vessel." .)

I've seen this clue in The Irish Times.

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