Trend determiner (4)
I believe the answer is:
poll
'trend determiner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for poll that I've seen before include "head-count" , "Votes cast at an election" , "Means of ascertaining public opinion with questions" , "Voting as at an election" , "Test of voting opinion" .)