Teachers incubate bird (8)
I believe the answer is:
nuthatch
'bird' is the definition.
(nuthatch is a kind of bird)
'teachers incubate' is the wordplay.
'teachers' becomes 'nut' (National Union of Teachers).
'incubate' becomes 'hatch' ('hatch' can be a synonym of 'incubate').
'nut'+'hatch'='NUTHATCH'
(Other definitions for nuthatch that I've seen before include "warbler?" , "small bird" , "Small songbird that climbs up and down trees looking for food" , "Tree-climbing songbird" , "Songbird named from its penchant for a kind of fruit" .)