Plant in reptile house guided upwards (8)
I believe the answer is:
asphodel
'plant' is the definition.
(type of flower)
'reptile house guided upwards' is the wordplay.
'reptile' becomes 'asp' (asp is a kind of reptile).
'house' becomes 'ho' (abbreviation for house).
'guided' becomes 'led' ('lead' can be a synonym of 'guide').
'upwards' shows that the letters should be reversed in order.
'led' reversed gives 'del'.
'asp'+'ho'+'del'='ASPHODEL'
'in' is the link.
(Other definitions for asphodel that I've seen before include "Flower of lily family - old heaps (anag)" , "Plant of lily family - lop heads (anag)" , "liliaceous plant" , "Lily-like plant, the flower of death to ancient Greeks" , "Lad hopes (anag) - flower" .)