Smooth support, a tiny bit short (6)
I believe the answer is:
legato
'smooth' is the definition.
(musical term)
'support a tiny bit short' is the wordplay.
'support' becomes 'leg' (leg is a kind of support).
'a tiny bit' becomes 'atom' (I've seen this before).
'short' means to remove the last letter.
'atom' with its final letter taken off is 'ato'.
'leg'+'ato'='LEGATO'
(Other definitions for legato that I've seen before include "play on without pause" , "Smoothly-flowing" , "Played smoothly" , "In a smooth and flowing manner (music)" , "(Music) without breaks between notes" .)