Tree inside Chelsea showground (3)
I believe the answer is:
ash
'tree' is the definition.
(ash tree is a kind of tree)
'inside chelsea showground' is the wordplay.
'inside' says the answer is hidden in the clue.
'ASH' is hidden within 'chelsea showground'.
(Other definitions for ash that I've seen before include "In fireplace it may be" , "Burnt material; tree" , "Grey colour" , "Runic letter; tree" , "Powdery residue from fire or cigarette" .)