Herb is sage, essentially, in unlimited clover (6)
I believe the answer is:
lovage
'herb' is the definition.
(lovage is a kind of herb)
'sage essentially in unlimited clover' is the wordplay.
'essentially' means to look at the middle letters.
'in' is an insertion indicator.
'unlimited' says to take the centre (discard the edges or 'limits' of the word).
The centre of 'sage' is 'ag'.
The middle of 'clover' is 'love'.
'ag' placed within 'love' is 'LOVAGE'.
'is' acts as a link.
(Other definitions for lovage that I've seen before include "F" , "Salad plant from a glove?" , "herb flavouring" , "Salad plant from southern Europe" , "Herb and salad plant" .)