Swallowing umpteenth tablet, unsettled in flight (2,3,4)
I believe the answer is:
on the wing
'in flight' is the definition.
(synonyms)
'swallowing umpteenth tablet unsettled' is the wordplay.
'swallowing' means one lot of letters goes inside another.
'umpteenth' becomes 'nth' (I've seen this before).
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'unsettled' becomes 'owing' (I've seen this before).
'nth'+'e'='nthe'
'nthe' placed into 'owing' is 'ON THE WING'.
(Other definitions for on the wing that I've seen before include "Where one used to see 7 or 11" , "in the air" , "Where feathers are" , "in flight?" , "flier" .)