Composer is boring second librettist (3,4)
I believe the answer is:
tim rice
'composer' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'boring second librettist' is the wordplay.
'boring' indicates putting letters inside (some letters must 'bore' a hole into other letters).
'second' becomes 'trice' (both can mean a very short amount of time).
'librettist' becomes 'im' (I can't explain this - if you can you should give a lot more credence to this answer).
'trice' placed around 'im' is 'TIM RICE'.
'is' is the link.
Can you help me to learn more?
(Another definition for tim rice that I've seen is " Chess writer".)