Reg must change to be with the bird (5)
I believe the answer is:
grebe
'bird' is the definition.
(type of bird)
'reg must change to be' is the wordplay.
'must change' indicates an anagram (I've seen this in other clues).
'reg' anagrammed gives 'gre'.
'gre'+'be'='GREBE'
'with the' acts as a link.
(Other definitions for grebe that I've seen before include "Freshwater diving bird" , "Aquatic bird that builds floating nests" , "Diving waterbird" , "Diver" , "Small diving waterfowl with a long neck" .)