A Greek character proved ultimately to be a pest (5)
I believe the answer is:
aphid
'pest' is the definition.
(I've seen this before)
'a greek character proved ultimately to be a' is the wordplay.
'a greek character' becomes 'phi' (letter from the Greek alphabet**).
'ultimately' suggests the final letters.
'to' means one lot of letters go next to another (I've seen this in other clues).
The final letter of 'proved' is 'd'.
'phi'+'d'='phid'
'phid' put after 'a' is 'APHID'.
(Other definitions for aphid that I've seen before include "Sap-sucking bug" , "Blackfly or greenfly" , "Horticultural pest" , "Garden pest" , "Small insect like greenfly" .)