Pray to tree, having taken hallucinogens (7)
I believe the answer is:
beseech
'pray' is the definition.
(I've seen this before)
'tree having taken hallucinogens' is the wordplay.
'tree' becomes 'beech' (beech is a kind of tree).
'having taken' indicates putting letters inside.
'hallucinogens' becomes 'es' (ecstasy tablets).
'beech' going around 'es' is 'BESEECH'.
'to' is the link.
(Other definitions for beseech that I've seen before include "Fervently ask (someone) for something" , "Sue" , "petition" , "Beg earnestly, plead" , "Beg earnestly, implore" .)