Cover I had left on top (3)
I believe the answer is:
lid
'cover' is the definition.
(I know that lid is a type of cover)
'i had left on top' is the wordplay.
'i had' becomes 'I'd' (contraction).
'left' becomes 'l' (common abbreviation).
'on top' means one lot of letters go next to another (in a down clue, some letters go above others).
'id' put after 'l' is 'LID'.
(Other definitions for lid that I've seen before include "Top for pot or jar" , "Cover from gliders" , "Removable top" , "Removable cover of a container" , "Bottle or jar top" .)