Fruit seed radius (4)
I believe the answer is:
pear
'fruit' is the definition.
(pear is a kind of fruit)
'seed radius' is the wordplay.
'seed' becomes 'pea' (I've seen this before**).
'radius' becomes 'r' (abbreviation in geometry).
'pea'+'r'='PEAR'
(Other definitions for pear that I've seen before include "Brilliant" , "Fruit; sounds like duo" , "Orchard fruit" , "Source of cider" , "Brand" .)