Only a balloon, thank goodness (4,2,4)
I believe the answer is:
just as well
'thank goodness' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'only a balloon' is the wordplay.
'only' becomes 'just' (synonyms).
'balloon' becomes 'swell' (I've seen this before).
'just'+'a'+'swell'='JUST AS WELL'
Can you help me to learn more?
(Other definitions for just as well that I've seen before include "A good thing" , "Fortunate" , "desirable!" .)