Lamp is not heavy (5)
I believe the answer is:
light
This clue is a double definition.
'lamp' is the first definition.
(I've seen this before)
'not heavy' is the second definition.
('light' is the opposite to 'heavy')
'is' acts as a link.
(Other definitions for light that I've seen before include "Weightless; lamp" , "Pre-eminent person; window" , "What smoker may be requesting" , "Easily lifted" , "Torch" .)