Producer of fruit: pulpy drink mostly contains it (6)
I believe the answer is:
citrus
'producer of fruit' is the definition.
(citrus plants produce fruit)
'pulpy drink mostly contains it' is the wordplay.
'pulpy drink' becomes 'crush' (crush is a type of fruit drink).
'mostly' means to remove the last letter.
'contains' indicates putting letters inside.
'crush' with its final letter taken away is 'crus'.
'crus' going around 'it' is 'CITRUS'.
(Other definitions for citrus that I've seen before include "Fruit from orange or lemon tree" , "Fruits such as oranges or lemons" , "Lemons, limes, oranges etc." , "sort of fruit tree" , "source of fruit" .)