Gradually destroy fuel element in reactor’s heart (7)
I believe the answer is:
corrode
'gradually destroy' is the definition.
(to corrode is to destroy over time)
'fuel element in reactor's heart' is the wordplay.
'fuel element' becomes 'rod' (rods used in nuclear power).
'in' is an insertion indicator.
'reactor's heart' becomes 'core' (heart of a nuclear reactor).
'rod' placed into 'core' is 'CORRODE'.
(Other definitions for corrode that I've seen before include "Growth in hazel buds" , "Eat away, rust" , "Cause to rust" , "Slowly eat" , "Chemically weaken" .)