Grub puts a lot of fat on Virginia (5)
I believe the answer is:
larva
'grub' is the definition.
(I know that grub can be written as larva)
'a lot of fat on virginia' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'fat' becomes 'lard' (lard is a kind of fat).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'virginia' becomes 'va' (US state abbreviation).
'lard' with its last letter taken off is 'lar'.
'lar'+'va'='LARVA'
'puts' is the link.
(Other definitions for larva that I've seen before include "Insect in the first post-egg stage of its life" , "immature six-footer?" , "early form" , "Insect just out of egg" , "Woodworm" .)