Swelling starts to build under Len's glass eye (5)
I believe the answer is:
bulge
'swelling' is the definition.
(I know that swelling can be written as bulge)
'starts to build under len's glass eye' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'build under len glass eye' is 'BULGE'.
(Other definitions for bulge that I've seen before include "Swell" , "Rounded swelling" , "Protrusion" , "Swelling - sudden increase" , "Hitler's offensive" .)