Bird which interrupts new union chapter (8)
I believe the answer is:
nuthatch
'bird' is the definition.
(nuthatch is a kind of bird)
'which interrupts new union chapter' is the wordplay.
'which' becomes 'that'.
'interrupts' means one lot of letters goes inside another.
'new' becomes 'n'.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'chapter' becomes 'ch'.
'n'+'u'+'ch'='nuch'
'that' inserted inside 'nuch' is 'NUTHATCH'.
(Other definitions for nuthatch that I've seen before include "Tree-climbing bird" , "small bird" , "[FEATHERED FRIEND]" , "Tree-climbing songbird" , "Climbing songbird" .)