Cheese being runny is enjoyable, primarily (4)
I believe the answer is:
brie
'cheese' is the definition.
(I know that Brie is a cheese)
'being runny is enjoyable primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'being runny is enjoyable' is 'BRIE'.
(Other definitions for brie that I've seen before include "Bergamot-flavoured tea" , "Soft, creamy French cheese" , "dairy product" , "Round soft Fr. cheese" , "Soft and creamy mild white cheese" .)