Secret I stumbled over - piece of plot (4)
I believe the answer is:
site
'plot' is the definition.
(as in a plot of land)
'secret i stumbled over piece' is the wordplay.
'piece' indicates the answer is hidden within the clue.
'over' says the letters should be written backwards.
'etis' is hidden in the letters of 'secret i stumbled'.
'etis' reversed gives 'SITE'.
'of' acts as a link.
(Other definitions for site that I've seen before include "station" , "Position; situation" , "Location (of building, dig etc)" , "Position for the building sounds visual" , "Building plot" .)