He was good, as most librettists can be (7)
I believe the answer is:
gilbert
'he was good as most librettists can be' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he was good as' is the wordplay.
'he' becomes 'Gil'.
'was good as' becomes 'bert' (I am not sure about this - if you are sure you should believe this answer much more).
'gil'+'bert'='GILBERT'
Can you help me to learn more?
(Other definitions for gilbert that I've seen before include "W. S. -, The Bab Ballads author" , "Sullivan's librettist" , "Sullivan's collaborator" , "Librettist for Ruddigore, d. 1911" , "formerly associated with Ellice" .)