Observe a defect (4)
I believe the answer is:
spot
This clue is a double definition.
'observe' is the first definition.
(I've seen this before)
'a defect' is the second definition.
(I know that spot is a type of defect)
(Other definitions for spot that I've seen before include "small amount" , "Observe - place" , "make out" , "awkward situation" , "Drop of rain" .)