No reason not to be miles up indeed in a balloon! (3,2,4)
I believe the answer is:
may as well
'no reason not to' is the definition.
(I've seen this before)
'miles up indeed in a balloon' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'indeed' becomes 'ay' ('ay' and 'aye' can mean yes).
'in' is an insertion indicator.
'balloon' becomes 'swell' (I've seen this before).
'ay' backwards is 'ya'.
'a'+'swell'='aswell'
'ya' placed into 'aswell' is 'ayaswell'.
'm'+'ayaswell'='MAY AS WELL'
'be' is the link.
(Other definitions for may as well that I've seen before include "No reason not to" , "for the whole of spring?" .)