Provide an excuse? Of course. Only too happy! (8)
I believe the answer is:
pleasure
'only too happy' is the definition.
I can't tell whether this definition defines the answer.
'provide an excuse? of course' is the wordplay.
'provide an excuse?' becomes 'plea' (I can't justify this - if you can you should give a lot more credence to this answer).
'of course' becomes 'sure' (I've seen this before).
'plea'+'sure'='PLEASURE'
Can you help me to learn more?
(Other definitions for pleasure that I've seen before include "'Enjoyment, delight (8)'" , "Feeling of enjoyment or satisfaction" , "Urgent entreaty begins gratification" , "Delectation, delight" , "A desirable emotion" .)