A balloon for good measure (2,4)
I believe the answer is:
as well
'for good measure' is the definition.
Both the answer and definition are adverbs. Maybe you can see an association between them that I don't see?
'a balloon' is the wordplay.
'balloon' becomes 'swell' (I've seen this in another clue).
'a'+'swell'='AS WELL'
Can you help me to learn more?
(Other definitions for as well that I've seen before include "A swell sort of addition" , "with equal effect" , "Also, in addition" , "Appropriate" , "To boot" .)