Ecstasy going into red wine as principle? (5)
I believe the answer is:
tenet
'principle?' is the definition.
(a tenet is a principle or value)
'ecstasy going into red wine' is the wordplay.
'ecstasy' becomes 'E' (informal term for the drug).
'going into' indicates putting letters inside.
'red wine' becomes 'tent' (type of Spanish wine).
'e' inserted into 'tent' is 'TENET'.
'as' is the link.
(Other definitions for tenet that I've seen before include "Item of belief" , "Doctrine held to be true" , "you may believe in it" , "Key principle" , "Belief - maxim" .)