Bird and dog unite (8)
I believe the answer is:
dovetail
'unite' is the definition.
(to join with a dovetail joint)
'bird and dog' is the wordplay.
'bird' becomes 'dove' (dove is a kind of bird).
'and' means one lot of letters go next to another.
'dog' becomes 'tail' (synonyms).
'dove'+'tail'='DOVETAIL'
(Other definitions for dovetail that I've seen before include "Wood joint" , "Joint of tenon and mortise" , "Fit closely together like white bird and appendage" , "good joint" , "Interlocking joint" .)