Garden pest a Greek character named, ultimately (5)
I believe the answer is:
aphid
'garden pest' is the definition.
(I've seen this before)
'a greek character named ultimately' is the wordplay.
'greek character' becomes 'phi' (letter from the Greek alphabet).
'ultimately' suggests the final letters.
The last letter of 'named' is 'd'.
'a'+'phi'+'d'='APHID'
(Other definitions for aphid that I've seen before include "Insect feeding on plant juices" , "Blackfly or greenfly" , "little sucker" , "one living on juices" , "plant pest" .)