Seen in Pimlico: Green Giant (4)
I believe the answer is:
ogre
'giant' is the definition.
(I know that ogre is a type of giant)
'seen in pimlico green' is the wordplay.
'seen in' indicates the answer is hidden within the clue.
'OGRE' is hidden within 'pimlico green'.
(Other definitions for ogre that I've seen before include "Giant of folklore who liked to eat humans" , "Wicked and inhuman person" , "stern one who inspires fear" , "Ugly giant" , "Monster, troll" .)