Musical instrument in shape? (8)
I believe the answer is:
triangle
I believe this clue is a double definition.
'musical instrument' is the first definition.
(triangle is a kind of musical instrument)
'shape?' is the second definition.
(triangle is a kind of shape)
'in' is the link.
(Other definitions for triangle that I've seen before include "It could be eternal" , "Three-sided figure - musical instrument" , "diamond cut in half?" , "complex romantic situation?" , "Percussion instrument of geometric origin" .)