What keeps still, extremely withdrawn? A canine offered in swap deal? (8)

'a canine offered in swap deal?' is the definition.
I can't judge whether this definition defines the answer.

'what keeps still extremely withdrawn?' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'still' becomes 'yet' (yet can mean still or so far).
'extremely' becomes 'too' (eg extremely expensive could mean too expensive).
'withdrawn?' says the letters should be written backwards.
'too' reversed gives 'oot'.
'yet'+'oot'='yetoot'
'eh' enclosing 'yetoot' is 'EYETOOTH'.

Can you help me to learn more?