Prickly character criticised nationalist answer on energy (7)
I believe the answer is:
echidna
'prickly character' is the definition.
(echidnas are prickly animals)
'criticised nationalist answer on energy' is the wordplay.
'criticised' becomes 'chid' (chiding is a kind of criticizing).
'nationalist' becomes 'n' (abbreviation in politics).
'answer' becomes 'a' (abbreviation eg in Q and A).
'on' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mch).
'chid'+'n'+'a'='chidna'
'chidna' put after 'e' is 'ECHIDNA'.
(Other definitions for echidna that I've seen before include "Spiny egg-laying Australian mammal" , "Had nice (anag) -- spiny anteater" , "Little animal" , "One preying on the workers" , "This insectivore" .)