Paul goes mad about border flower (7)
I believe the answer is:
primula
'flower' is the definition.
(I know that Primula is a type of flower)
'paul goes mad about border' is the wordplay.
'goes mad' indicates an anagram.
'about' indicates putting letters inside.
'border' becomes 'rim' (both can mean an edge).
'paul' with letters rearranged gives 'pula'.
'pula' enclosing 'rim' is 'PRIMULA'.
(Other definitions for primula that I've seen before include "Cowslip, perhaps" , "Air plum, oddly, from this plant" , "Flowering plant" , "Cowslip genus" , "Primrose" .)