Cooked slab has this third herb (5)
I believe the answer is:
basil
'herb' is the definition.
(basil is a herb)
'cooked slab has this third' is the wordplay.
'cooked' is an anagram indicator (letters cooked into a new form).
'has' means one lot of letters goes inside another.
'this third' becomes 'i' (3rd letter of 'this').
'slab' anagrammed gives 'basl'.
'basl' placed around 'i' is 'BASIL'.
(Other definitions for basil that I've seen before include "Herb - Mr Fawlty" , "Culinary plant" , "named Cardinal Hume" , "Bails out the herb" , "Herb - boy's name" .)