My God, it's tops to be a dog! (5)
I believe the answer is:
corgi
'be a dog' is the definition.
(I've seen this before)
'my god it's tops' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'tops' suggests taking the first letters.
The initial letters of 'god its' is 'gi'.
'cor'+'gi'='CORGI'
'to' is the link.
(Other definitions for corgi that I've seen before include "Cardigan, possibly" , "Queen likes this" , "Welsh breen of dog" , "Welsh breed of short-legged dog" , "Dog popular with British royals" .)