Elemental form is uppermost in poet's heart (7)
I believe the answer is:
isotope
'elemental form' is the definition.
(variant or form of an element)
'is uppermost in poet's heart' is the wordplay.
'uppermost' becomes 'top' (similar in meaning).
'in' means one lot of letters goes inside another.
'poet's' becomes 'poes' (Edgar Allan Poe).
'heart' says to take the centre.
The central letters of 'poes' are 'oe'.
'top' inserted within 'oe' is 'otope'.
'is'+'otope'='ISOTOPE'
(Other definitions for isotope that I've seen before include "Variant element form" , "Form of atomic nucleus" , "One of the variants of an element sharing an atomic number but with differing chemical properties" , "Atom with the same atomic number as another but with different numbers of neutrons" , "elementary variation in it" .)