Firmly encouraging going after supply oddly and being very extravagant (9)

'being very extravagant' is the definition.
I can't tell whether this defines the answer.

'firmly encouraging going after supply oddly' is the wordplay.
'firmly encouraging' becomes 'urging' (urging is a kind of encouraging. I am not sure about the 'firmly' bit.).
'going after' is a charade indicator (letters next to each other).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'supply' are 'spl'.
'urging' after 'spl' is 'SPLURGING'.

'and' acts as a link.

Can you help me to learn more?