With sitar periodically playing sing out forte (6,4)
I believe the answer is:
strong suit
'forte' is the definition.
(I've seen this in another clue)
'with sitar periodically playing sing out' is the wordplay.
'with' means one lot of letters go next to another.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'playing' indicates anagramming the letters.
The alternating letters of 'sitar' are 'str'.
'sing'+'out'='singout'
'singout' with letters rearranged gives 'ongsuit'.
'str'+'ongsuit' is 'STRONG SUIT'.
(Other definitions for strong suit that I've seen before include "special talent" , "Something at which one excels" .)