Prepare the brain to conserve energy? (9)
I believe the answer is:
hibernate
'conserve energy?' is the definition.
'prepare the brain to' is the wordplay.
'prepare' indicates anagramming the letters.
'to' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'the'+'brain'='thebrain'
'thebrain' with letters rearranged gives 'hibernat'.
'hibernat'+'e'='HIBERNATE'
(Other definitions for hibernate that I've seen before include "remain inactive for months" , "Pass the winter in a dormant state" , "Sleep durning winter" , "Breathe in for long winter sleep" , "go dormant" .)