Tends to drop a glass (4)
I believe the answer is:
lens
'glass' is the definition.
(I've seen this before)
'tends to drop a' is the wordplay.
'tends' becomes 'leans' ('lean' can be a synonym of 'tend').
'to drop' suggests deleting specific letters.
'leans' with 'a' taken away is 'LENS'.
(Other definitions for lens that I've seen before include "Light-gathering device of a camera" , "One could project a picture" , "Light-focusing device" , "Optical device using light to form images" , "Focussing glass" .)