In miniature, because an instrument (5)
I believe the answer is:
rebec
'instrument' is the definition.
(a rebec is an old stringed instrument)
'in miniature because an' is the wordplay.
'in' says the answer is hidden in the clue.
'REBEC' is hidden within 'miniature because an'.
(Other definitions for rebec that I've seen before include "Old stringed instrument" , "Mediaeval stringed instrument" , "Medieval bowed instrument" .)