Experts in nonpoetic language, mostly (4)
I believe the answer is:
pros
'experts' is the definition.
(professionals)
'nonpoetic language mostly' is the wordplay.
'nonpoetic language' becomes 'prose' (as opposed to poetry).
'mostly' means to remove the last letter (most but not all of the word).
'prose' with its last letter taken away is 'PROS'.
'in' is the link.
(Other definitions for pros that I've seen before include "Yes votes" , "Sportsmen who are paid" , "Arguments in favour" , "Opposites of cons" , "Persons with knowledge and skill" .)