Pretentious fellow geek taking Ecstasy (8)
I believe the answer is:
mannered
'pretentious' is the definition.
(thesaurus)
'fellow geek taking ecstasy' is the wordplay.
'fellow' becomes 'man' (I've seen this before**).
'geek' becomes 'nerd' (I've seen this before).
'taking' is an insertion indicator.
'ecstasy' becomes 'e' (informal term for the drug).
'nerd' placed around 'e' is 'nered'.
'man'+'nered'='MANNERED'
(Other definitions for mannered that I've seen before include "Artificial, stilted" , "Given to polite social behaviour" , "unnatural" , "Acting with exaggerated behaviour" , "Affected, stilted" .)