Herb tells the hour (5)
I believe the answer is:
thyme
'herb' is the definition.
(I know that thyme is a type of herb)
'tells the hour' is the wordplay.
'tells' shows a homophone (sound like) (I've seen 'told' mean this).
'the hour' becomes 'time' (I've seen this in another clue).
'time' is a homophone of 'THYME'.
(Other definitions for thyme that I've seen before include "Aromatic plant" , "Small shrub: leaves used to season food" , "Shrub - herb" , "Minty herb" , "Ingredient in cooking" .)