Birds ultimately flaunt it over rodents (7)
I believe the answer is:
titmice
'birds' is the definition.
(titmouse is a kind of bird)
'ultimately flaunt it over rodents' is the wordplay.
'ultimately' indicates one should take the final letters.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'rodents' becomes 'mice' (mouse is a kind of rodent).
The final letter of 'flaunt' is 't'.
't'+'it'+'mice'='TITMICE'
(Other definitions for titmice that I've seen before include "Small birds" , "little birds" , "Little singers" , "Small, active songbirds" .)