Worker in grounds is finally sent to stop horse rearing (7)
I believe the answer is:
barista
'worker in grounds' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'is finally sent to stop horse rearing' is the wordplay.
'finally' suggests the final letters.
'to stop' means one lot of letters goes inside another (stop can mean to plug a hole in something).
'horse' becomes 'arab' (Arab is a type of horse).
'rearing' says the letters should be written backwards (as in a horse rearing up).
The last letter of 'sent' is 't'.
'arab' in reverse letter order is 'bara'.
'is'+'t'='ist'
'ist' put inside 'bara' is 'BARISTA'.
Can you help me to learn more?
(Other definitions for barista that I've seen before include "person serving drinks" , "Coffee expert" , "coffee-maker" , "Coffee maker?" , "Arabist (anag)" .)