Really expressing agreement about kiwi's wings (2,7)

'really' is the definition.

'agreement about kiwi's wings' is the wordplay.
'agreement' becomes 'nodding' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'wings' means to remove the middle letters (the wings or edge letters of the word are used).
'kiwi' with its middle taken out is 'ki'.
'nodding' placed around 'ki' is 'NO KIDDING'.

'expressing' acts as a link.

Can you help me to learn more?