Moonlight, perhaps working in smart bar periodically (6)
I believe the answer is:
sonata
'moonlight perhaps' is the definition.
(I know that Piano Sonata No. 14 is a piano sonata)
'working in smart bar periodically' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'in' indicates putting letters inside.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'smart'+'bar'='smartbar'
The alternate letters of 'smartbar' are 'sata'.
'on' put into 'sata' is 'SONATA'.
(Other definitions for sonata that I've seen before include "musical performance" , "Music for a soloist" , "Musical composition with three or more contrasting movements" , "Piece of classical music" , "Musical work for a solo instrument" .)