Firm chaff initially swallowed by mountain bird (7)
I believe the answer is:
peacock
'bird' is the definition.
(peacock is a kind of bird)
'firm chaff initially swallowed by mountain' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'initially' says to take the initial letters.
'swallowed by' means one lot of letters goes inside another.
'mountain' becomes 'peak' (I've seen this before).
The first letter of 'chaff' is 'c'.
'co'+'c'='coc'
'coc' going inside 'peak' is 'PEACOCK'.
(Other definitions for peacock that I've seen before include "It has very long tail feathers that have iridescent eye-like markings" , "Showy fowl" , "Blue-green" , "Thomas Love --, author" , "Bird noted for showy plumage" .)