It's a sort of bird, we affirm (6)
I believe the answer is:
weaver
'it's a sort of bird' is the definition.
'we affirm' is the wordplay.
'affirm' becomes 'aver' ('aver' can be a synonym of 'affirm').
'we'+'aver'='WEAVER'
(Other definitions for weaver that I've seen before include "One who makes fabric with loom etc" , "Bottom's trade (MND)" , "Loom worker; tropical bird" , "Bottom was one (MND)" , "Fabric maker" .)