Glass fallen, smashed, though not entirely (4)
I believe the answer is:
lens
'glass' is the definition.
(I've seen this before)
'fallen smashed though not entirely' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'LENS' is hidden within 'fallen smashed though'.
(Other definitions for lens that I've seen before include "Camera component" , "City of northern France: periscope part" , "French city" , "Light-focusing device" , "optical equipment" .)