Stand next to instrument that's spinning (4)
I believe the answer is:
abut
'stand next to' is the definition.
(I've seen this in another clue)
'instrument that's spinning' is the wordplay.
'instrument' becomes 'tuba' (tuba is a kind of instrument).
'that's spinning' says the letters should be written backwards.
'tuba' written backwards gives 'ABUT'.
(Other definitions for abut that I've seen before include "Be next to -- share a boundary" , "Adjoin, border" , "Lie next to" , "Adjoin, press up against" , "Meet" .)