Drone blocked by extension on current instrument (9)
I believe the answer is:
harmonium
'instrument' is the definition.
(harmonium is a kind of instrument)
'drone blocked by extension on current' is the wordplay.
'drone' becomes 'hum' (drone can mean to make a humming sound).
'blocked by' is an insertion indicator (I've seen this in other clues).
'extension' becomes 'arm' (I've seen this in another clue).
'current' becomes 'I' (symbol for electric current).
'arm'+'on'+'i'='armoni'
'hum' placed around 'armoni' is 'HARMONIUM'.
(Other definitions for harmonium that I've seen before include "Keyboard instrument - a minor hum (anag)" , "Pedalled-bellows instrument" .)