Stringed instruments played while laid flat (7)
I believe the answer is:
zithers
'stringed instruments played while laid flat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for zithers that I've seen before include "Stringed instruments played while laid flat" , "Note producers" , "Musical instruments" , "Stringed instruments plucked when lying flat" .)