Form of an element (7)
I believe the answer is:
isotope
'form of an element' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for isotope that I've seen before include "O, is poet involved in chemical element?" , "Form of atomic nucleus" , "elementary variation in it" , "Oops, tie up the atom" , "O, is poet a species of atom?" .)