Plant key behind reptile house (8)
I believe the answer is:
asphodel
'plant' is the definition.
(type of flower)
'key behind reptile house' is the wordplay.
'key' becomes 'del' (del key on a keyboard).
'behind' is a charade indicator (letters next to each other).
'reptile' becomes 'asp' (asp is a kind of reptile).
'house' becomes 'ho' (abbreviation for house).
'asp'+'ho'='aspho'
'del' put after 'aspho' is 'ASPHODEL'.
(Other definitions for asphodel that I've seen before include "Shop deal (anag.)" , "Lad hopes (anag) - flower" , "blooms from Elysian fields" , "liliaceous plant" , "Plant of lily family - lop heads (anag)" .)