Bird, one good in open flying (6)
I believe the answer is:
pigeon
'bird' is the definition.
(type of bird)
'one good in open flying' is the wordplay.
'one' becomes 'i' (Roman numeral).
'good' becomes 'g' (abbreviation).
'in' indicates putting letters inside.
'flying' indicates an anagram (letters fly around into new positions).
'i'+'g'='ig'
'open' anagrammed gives 'peon'.
'ig' put inside 'peon' is 'PIGEON'.
(Other definitions for pigeon that I've seen before include "Baby" , "run, perhaps" , "Homer, possibly" , "Homing bird" , "Epigon (anag.)" .)