Sleep sees the brain changing shape, ultimately (9)
I believe the answer is:
hibernate
'sleep' is the definition.
(hibernating is a kind of sleeping)
'the brain changing shape ultimately' is the wordplay.
'changing' indicates an anagram.
'ultimately' suggests the final letters.
The final letter of 'shape' is 'e'.
'the'+'brain'='thebrain'
'thebrain' anagrammed gives 'hibernat'.
'hibernat'+'e'='HIBERNATE'
'sees' acts as a link.
(Other definitions for hibernate that I've seen before include "Pass the winter in a dormant state" , "Breathe in for long winter sleep" , "Of animals, sleep through the winter" , "go dormant" , "go out in the snow?" .)