Two under is a birdie (5)
I believe the answer is:
eagle
This clue is a double definition.
'two under' is the first definition.
(I've seen this in another clue)
'birdie' is the second definition.
(thesaurus)
'is a' is the link.
(Other definitions for eagle that I've seen before include "Keen-eyed bird" , "The bald one is a US symbol" , "drugs with silver left inside" , "somewhat under par" , "Old comic" .)