Brainless creature Obama replaced absorbs energy (6)
I believe the answer is:
amoeba
'brainless creature' is the definition.
(single-cell lifeform)
'obama replaced absorbs energy' is the wordplay.
'replaced' indicates an anagram.
'absorbs' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'obama' with letters rearranged gives 'amoba'.
'amoba' placed around 'e' is 'AMOEBA'.
(Other definitions for amoeba that I've seen before include "Basic life form" , "insignificant creature" , "O, Mam, be a one-celled animal" , "Microscopic organism consisting of a single cell changing constantly" , "Protozoon" .)