Unit of nuclear energy? (3)
I believe the answer is:
are
'unit' is the definition.
(are is a kind of unit)
'of nuclear energy?' is the wordplay.
'of' says the answer is hidden in the clue.
'ARE' can be found hidden inside 'nuclear energy'.
(Other definitions for are that I've seen before include "Is, in the plural" , "live" , "(We/you/they) exist" , "is for more than one" , "(We/they/you) exist" .)