Gas somewhat dear, go nuclear (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'somewhat dear go nuclear' is the wordplay.
'somewhat' indicates a hidden word.
'ARGON' is hidden within 'dear go nuclear'.
(Other definitions for argon that I've seen before include "Gas used in electric lights" , "17 [GAS]" , "Gaseous element" , "An insert gas to groan about" , "something inert and insubstantial" .)