A number rendered by very good instrument (5)
I believe the answer is:
piano
'instrument' is the definition.
(musical instrument)
'a number rendered by very good' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'rendered by' is a charade indicator (letters next to each other).
'very good' becomes 'pi' (old-fashioned term for 'pious').
'a'+'no'='ano'
'ano' after 'pi' is 'PIANO'.
(Other definitions for piano that I've seen before include "Direction in music (1): relatively softly" , "Instrument with ivories" , "Franz Liszt's instrument" , "one of 6 [STEINWAYS]" , "Softly (mus.)" .)