Fruit seed radius (4)
I believe the answer is:
Here is my best explanation:
'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).
(Other definitions for pear that I've seen before include "One fruit sounds like two", "Alternative to apple", "Conference fruit?", "Tasty fruit that tapers towards the stalk", "Source of cider", "Fruit related to apple", "Tasty fruit to reap", "Tapering fleshy fruit - reap it", "Pomaceous fruit" and "Cone-shaped fruit".)