Reduced in size (4)
(Other definitions for less that I've seen before include "Comparative of 'a little'" , "Fewer in number" , "Fewer; minus" , "In smaller quantity" , "inferior" .)
'reduced in size' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for shrank that I've seen before include "Shrivelled" , "Became smaller" , "Recoiled" , "Contracted, grew smaller" , "Reduced in size" .)