Stone found in grub -- yuck! (4)
I believe the answer is:
ruby
'stone' is the definition.
(I've seen this before)
'in grub yuck' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RUBY' can be found hidden inside 'grub yuck'.
'found' acts as a link.
(Other definitions for ruby that I've seen before include "Deep red variety of corundum" , "Completely" , "Vivid red" , "Gemstone - shade of red" , "Fortieth anniversary stone" .)