Nettle tea ruined by stirring centrally over time (8)
I believe the answer is:
irritate
'nettle' is the definition.
('irritate' can be a synonym of 'nettle')
'tea ruined by stirring centrally over time' is the wordplay.
'ruined' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'centrally' indicates the central letters.
'over' says to put letters next to each other.
'time' becomes 't' (**).
The central letters of 'stirring' are 'irri'.
'tea' is an anagram of 'ate'.
'irri'+'t'='irrit'
'ate' put after 'irrit' is 'IRRITATE'.
(Other definitions for irritate that I've seen before include "'Annoy, vex (8)'" , "badger" , "Wind up" , "Annoy or cause itching" , "Cause annoyance" .)