An old Labour leader in the air (5)
I believe the answer is:
afoot
'in the air' is the definition.
(I've seen this before)
'an old labour leader' is the wordplay.
'an' becomes 'a'.
'old labour leader' becomes 'Foot' (Michael Foot, former leader of the Labour Party).
'a'+'foot'='AFOOT'
(Other definitions for afoot that I've seen before include "In the pipeline" , "Stirring" , "Making progress" , "Currently in progress" , "Happening" .)