Piece of cake catching large seal (6)
I believe the answer is:
clinch
'seal' is the definition.
(I've seen this before)
'piece of cake catching large' is the wordplay.
'piece of cake' becomes 'cinch' ('cinch' can be a synonym of 'piece of cake').
'catching' is an insertion indicator.
'large' becomes 'l'.
'cinch' going around 'l' is 'CLINCH'.
(Other definitions for clinch that I've seen before include "Grapple - confirm" , "Fierce embrace" , "Clutch" , "Confirm - embrace" , "Passionate embrace" .)