Trends get fellow terribly sad (4)
I believe the answer is:
fads
'trends' is the definition.
(fad is a kind of trend)
'get fellow terribly sad' is the wordplay.
'get' is a charade indicator (letters next to each other).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'terribly' indicates an anagram.
'sad' with letters rearranged gives 'ads'.
'f' next to 'ads' is 'FADS'.
(Other definitions for fads that I've seen before include "Fashions or vogues" , "Short-lived crazes" .)