Begin attaching balloon to boot (2,4)
I believe the answer is:
as well
'to boot' is the definition.
(I've seen this before)
'begin attaching balloon' is the wordplay.
'begin' says to take the initial letters.
'balloon' becomes 'swell' (I've seen this in another clue).
The initial letter of 'attaching' is 'a'.
'a'+'swell'='AS WELL'
(Other definitions for as well that I've seen before include "To boot" , "Also, in addition" , "on top" , "A swell sort of addition" , "with equal effect" .)