Stage action, oddly smooth (6)
I believe the answer is:
legato
'smooth' is the definition.
('legato' can be similar in meaning to 'smooth')
'stage action oddly' is the wordplay.
'stage' becomes 'leg' ('leg' can be a synonym of 'stage').
'oddly' means one should take alternating letters.
The alternating letters of 'action' are 'ato'.
'leg'+'ato'='LEGATO'
(Other definitions for legato that I've seen before include "(Music) without breaks between notes" , "Smoothly, musically speaking" , "In music, in a smooth even manner" , "Don't play roughly" , "Musically smooth" .)