Mature herb needs head pruning (3)
I believe the answer is:
age
'mature' is the definition.
(synonyms)
'herb needs head pruning' is the wordplay.
'herb' becomes 'sage' (sage is a kind of herb).
'needs head pruning' means to remove the first letter.
'sage' with its initial letter taken away is 'AGE'.
(Other definitions for age that I've seen before include "How long something has existed" , "Just a number" , "Maturity" , "Get old" , "Grow older" .)