Bird which interrupts new union chapter (8)
I believe the answer is:
Here is my best explanation:
'bird' is the definition.
(nuthatch is a kind of bird)
'which interrupts new union chapter' is the subsidiary indication.
'which' becomes 'that'.
'interrupts' is an insertion indicator.
'new' becomes 'n'.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'chapter' becomes 'ch'.
'that' put into '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", "Trunk-climbing bird", "warbler?", "Flier", "Bird - hunt chat (anag)" and "Songbird named from its penchant for a kind of fruit".)