Behold gate moving inside smoothly (6)
I believe the answer is:
legato
'smoothly' is the definition.
(I've seen this before)
'behold gate moving inside' is the wordplay.
'behold' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'moving' indicates anagramming the letters.
'inside' is an insertion indicator.
'gate' is an anagram of 'egat'.
'lo' enclosing 'egat' is 'LEGATO'.
(Other definitions for legato that I've seen before include "scored smoothly" , "Don't play roughly" , "In music, in a smooth even manner" , "Smoothly-flowing" , "play on without pause" .)