Old northern Conservative with ecstasy a while back (4)
I believe the answer is:
once
'a while back' is the definition.
(once can mean a while back in time)
'old northern conservative with ecstasy' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'conservative' becomes 'C' (UK political abbreviation).
'with' says to put letters next to each other.
'ecstasy' becomes 'E' (informal term for the drug).
'o'+'n'+'c'+'e'='ONCE'
(Other definitions for once that I've seen before include "Cone (anag.)" , "Some time back" , "At a previous time" , "in bygone days" , "Just as a single occurrence" .)