Bird's head to emerge from shell (8)
I believe the answer is:
nuthatch
'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).
'nut'+'hatch'='NUTHATCH'
(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" .)