Bird's head to emerge from shell (8)
I believe the answer is:
'bird's' is the definition.
(nuthatch is a kind of bird)
'head to emerge from shell' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'to emerge from shell' becomes 'hatch' (I've seen this before).
(Other definitions for nuthatch that I've seen before include "Tree-climbing songbird" , "[FEATHERED FRIEND]" , "Singer" , "Bird - hunt chat (anag)" , "Small songbird often seen on tree trunks" .)