Bird raced around very energetically at first (5)
I believe the answer is:
raven
'bird' is the definition.
(raven is a kind of bird)
'raced around very energetically at first' is the wordplay.
'raced' becomes 'ran' ('run' can be a synonym of 'race').
'around' means one lot of letters goes inside another.
'at first' indicates taking the first letters.
The first letters of 'very energetically' is 've'.
'ran' going around 've' is 'RAVEN'.
(Other definitions for raven that I've seen before include "Species of crow" , "Flighty type" , "Tower of London bird" , "Large black bird" , "flesh-eating bird" .)