Fan chasing diminished top seed (6)
I believe the answer is:
peanut
'seed' is the definition.
(I know that peanut is a type of edible nut)
'fan chasing diminished top' is the wordplay.
'fan' becomes 'nut' (both can mean an enthusiast).
'chasing' means one lot of letters go next to another (some letters chase after others).
'diminished' means to remove the last letter.
'top' becomes 'peak' (synonyms).
'peak' with its final letter taken off is 'pea'.
'nut' put after 'pea' is 'PEANUT'.
(Other definitions for peanut that I've seen before include "Something to eat" , "Item of fruit" , "Bar nibble" , "Singularly little money" , "Satay ingredient" .)