The most common and most likely answer to this crossword clue is the 6 letter word TOMTIT
I've seen this crossword clue in The Guardian
T
O
M
T
I
T
More clues leading to the result TOMTIT
- Singer reversed test, having ultimately scant appeal
- Initiation of this Italian test upset singer
- Bird — and cat beginning to track it
- Bird breast taken by cat
- After tango, miss catching heel of cabaret singer
- Bird – cat finally caught it
- ‘Pussy and boob’ sort of bird
- Regular visitor to feeder? Cat finally caught it
- Bird, male goose, losing weight
- Cat dug to find bird
Buddy explains!

The straight part
We know that "Bird" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bird = tomtit.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TOMTIT mean?
TOMTIT noun- Petroica macrocephala, the miromiro or New Zealand tit, a bird of the Petroicidae (Australasian robin) family.