Staff do nothing about instrument (8)
I believe the answer is:
Here is my best explanation:
'instrument' is the definition.
(mandolin is a kind of instrument)
'staff do nothing about' is the subsidiary indication.
'staff' becomes 'man' (manning is a kind of staffing).
'do' becomes 'do' (**).
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'about' shows that the letters should be reversed in order.
'nil' reversed gives 'lin'.
(Other definitions for mandolin that I've seen before include "Almond in the stringed instrument, oddly", "one's deployed in the kitchen", "vegetable slicer", "Instrument resembling a lute", "Round-backed musical instrument", "it has strings attached", "Small guitar-like instrument", "Musical instrument like a small guitar", "Plucked instrument" and "Musical instrument like a small lute".)