Be sickly sweet (4)
I believe the answer is:
cloy
'be sickly sweet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cloy that I've seen before include "Cause distaste by being too sweet or rich" , "overindulge" , "Make weary through being overly sweet" , "Make weary through over-sentimentality" , "Sicken by glutting" .)