Club's last, equally true voice (5)
I believe the answer is:
'voice' is the definition.
(I know that basso is a type of singing voice)
'club's last equally true' is the wordplay.
'last' says to take the final letters.
'equally' becomes 'as' (synonyms).
'true' becomes 'so' (similar in meaning).
The final letter of 'club' is 'b'.
(Other definitions for basso that I've seen before include "low voice" , "Singer" , "one's deep in sound" , "Lowest male singing voice (sometimes 'profundo')" .)