Big beast in union, a dog barking (9)
I believe the answer is:
iguanodon
'big beast' is the definition.
(I've seen this before)
'union a dog barking' is the wordplay.
'barking' indicates an anagram (UK slang for mad).
'union'+'a'+'dog'='unionadog'
'unionadog' with letters rearranged gives 'IGUANODON'.
'in' is the link.
(Other definitions for iguanodon that I've seen before include "Long-tailed bipedal dinosaur" , "reptile now extinct" , "big beast" , "In the past a massive herbivore" , "Massive dinosaur of the early Cretaceous period" .)