Bird one’s seen in the flesh (6)
I believe the answer is:
siskin
'bird' is the definition.
(siskin is a kind of bird)
'one's seen in the flesh' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'seen in' is an insertion indicator.
'the flesh' becomes 'skin' (flesh is one's skin).
'is' inserted inside 'skin' is 'SISKIN'.
(Other definitions for siskin that I've seen before include "Yellowish-green finch" , "Finch akin to canary" , "Yellow-and-black finch" , "Type of finch" , "Small songbird related to the goldfinch" .)