Fit dog chasing bird (8)
I believe the answer is:
dovetail
'fit' is the definition.
(I know that dovetail is a kind of fit)
'dog chasing bird' is the wordplay.
'dog' becomes 'tail' (synonyms).
'chasing' means one lot of letters go next to another (some letters chase after others).
'bird' becomes 'dove' (dove is a kind of bird).
'tail' after 'dove' is 'DOVETAIL'.
(Other definitions for dovetail that I've seen before include "Interlocking joint" , "Fit snugly" , "good joint" , "Violated (anag) -- mortise joint" , "Type of joint - fit closely together" .)