A piece of cake is lardy, lacking in grain (4)
I believe the answer is:
easy
'a piece of cake' is the definition.
(I've seen this before)
'lardy lacking in grain' is the wordplay.
'lardy' becomes 'greasy'.
'lacking in' suggests deleting specific letters.
'grain' becomes 'gr' (short for grain, a unit of mass).
'greasy' with 'gr' taken out is 'EASY'.
'is' is the link.
(Other definitions for easy that I've seen before include "Not difficult" , "for a piece of cake" , "Gentle - light" , "Presenting few difficulties" , "Frederick Marryat's midshipman" .)