Cover up large herb (4)
I believe the answer is:
dill
'herb' is the definition.
(I know that Dill is a type of herb)
'cover up large' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'large' becomes 'L' (eg in clothes sizes).
'lid' in reverse letter order is 'dil'.
'dil'+'l'='DILL'
(Other definitions for dill that I've seen before include "Aromatic herb used as seasoning" , "Medicinal herb" , "Herb for gravadlax" , "Herb; rhymes with beak" , "Herb commonly used with gravlax" .)