A distended shape to boot (2,4)
I believe the answer is:
as well
'to boot' is the definition.
(I've seen this before)
'a distended shape' is the wordplay.
'distended shape' becomes 'swell' (a swollen shape).
'a'+'swell'='AS WELL'
(Other definitions for as well that I've seen before include "Appropriate" , "A swell sort of addition" , "Also, in addition" , "To boot" , "Also, additionally" .)