Behold one getting advanced instrument (5)
I believe the answer is:
viola
'instrument' is the definition.
(viola is a kind of instrument)
'behold one getting advanced' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'advanced' could be 'a' (abbreviation) and 'a' is present in the answer.
The remaining letters 'vol' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for viola that I've seen before include "one used to bowing?" , "Page" , "Shakespearean character" , "Large fiddle" , "big fiddle!" .)