A balloon in addition (2,4)

I believe the answer is:

as well

'in addition' is the definition.
(I've seen this before)

'a balloon' is the wordplay.
'balloon' becomes 'swell' (I've seen this in another clue).
'a'+'swell'='AS WELL'

(Other definitions for as well that I've seen before include "To boot" , "A swell sort of addition" , "Also, in addition" , "on top" , "with equal effect" .)

I've seen this clue in The Sun.

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