Turn over pest's troublesome complaint (5)
I believe the answer is:
upset
'complaint' is the definition.
(I've seen this before)
'turn over pest's troublesome' is the wordplay.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'troublesome' is an anagram indicator.
'pest' anagrammed gives 'pset'.
'u'+'pset'='UPSET'
(Other definitions for upset that I've seen before include "thrown?" , "dismay" , "Aggrieved" , "Disturb - spoil" , "Overturn; unhappy" .)