To start with be embarrassed by the early enthusiasm (5,5)

'early enthusiasm' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'to start with be embarrassed by' is the wordplay.
'to start' becomes 'first' (synonyms).
'with' means one lot of letters go next to another.
'be embarrassed by' becomes 'flush' (I can't justify this - if you can you should give a lot more credence to this answer).
'first'+'flush'='FIRST FLUSH'

'the' acts as a link.

Can you help me to learn more?