Drink for a bird-dog (8)
I believe the answer is:
cocktail
'drink' is the definition.
(cocktail is a kind of drink)
'a bird-dog' is the wordplay.
'a bird' becomes 'cock' (cock is a kind of bird).
'dog' becomes 'tail' (synonyms).
'cock'+'tail'='COCKTAIL'
'for' acts as a link.
(Other definitions for cocktail that I've seen before include "Possibly old-fashioned" , "dish with prawn" , "Mixed alcoholic drink or prawn appetiser" , "Manhattan, for example" , "Screwdriver, for instance" .)