Grab what is ultimately large in size (5)
I believe the answer is:
seize
'grab' is the definition.
(I know this)
'ultimately large in size' is the wordplay.
'ultimately' suggests the final letters.
'in' indicates putting letters inside.
The last letter of 'large' is 'e'.
'e' going into 'size' is 'SEIZE'.
'what is' is the link.
(Other definitions for seize that I've seen before include "Grab suddenly" , "Nobble" , "Snatch" , "Grab by force" , "Grasp forcibly" .)