Filling in maestro’s informal application for cellist (5)
I believe the answer is:
rosin
'application for cellist' is the definition.
(resin applied to string instruments)
'filling in maestro's informal' is the wordplay.
'filling in' indicates a hidden word.
'ROSIN' is hidden within 'maestros informal'.
(Other definitions for rosin that I've seen before include "It's put on bow" , "Fiddler's requirement" , "It's used on a violin bow" , "Wax for violin bow etc." , "Fiddler's resin" .)