Trendy, yes, developing an infection (9)
I believe the answer is:
dysentery
'an infection' is the definition.
(I've seen this before)
'trendy yes developing' is the wordplay.
'developing' is an anagram indicator (letters developed into a new form).
'trendy'+'yes'='trendyyes'
'trendyyes' with letters rearranged gives 'DYSENTERY'.
(Other definitions for dysentery that I've seen before include "Infectious intestinal disease" , "Intestinal infection" .)