Criminal's priors oddly preceding Ecstasy scheme (8)
I believe the answer is:
conspire
'scheme' is the definition.
(I've seen this before)
'criminal's priors oddly preceding ecstasy' is the wordplay.
'criminal's' becomes 'cons' (abbreviation for 'convict').
'oddly' indicates alternate letters.
'preceding' means one lot of letters go next to another.
'ecstasy' becomes 'e' (informal term for the drug).
The alternate letters of 'priors' are 'pir'.
'cons'+'pir'+'e'='CONSPIRE'
(Other definitions for conspire that I've seen before include "Plot secretly" , "scheme" , "'Plot, connive (8)'" , "Collude" , "Secretly plan together" .)