Type of atom: is oxygen best to get energy? (7)
I believe the answer is:
isotope
'type of atom' is the definition.
(I've seen this before)
'is oxygen best to get energy?' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'to get' is a charade indicator (letters next to each other).
'energy?' becomes 'e' (physics symbol as in E=mc2).
'is'+'o'+'top'+'e'='ISOTOPE'
(Other definitions for isotope that I've seen before include "may be radioactive" , "Type of atom, maybe radioactive" , "Distinct form of an element" , "Oo, spite! (anag.)" , "O, is poet involved in chemical element?" .)