Energy entering a crowd, a tiny shape-shifter (6)
I believe the answer is:
amoeba
'tiny shape-shifter' is the definition.
(I've seen this in another clue)
'energy entering a crowd a' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mco).
'entering' means one lot of letters goes inside another.
'crowd' becomes 'mob' (mob can mean a crowd of people).
'a'+'mob'='amob'
'e' inserted within 'amob' is 'amoeb'.
'amoeb'+'a'='AMOEBA'
(Other definitions for amoeba that I've seen before include "Protozoon" , "insignificant creature" , "O, Mam, be a one-celled animal" , "Single-celled animal" , "parasitic type" .)