Being immature, turn up on ecstasy to provoke anger (7)
I believe the answer is:
Here is my best explanation:
'being immature' is the definition.
('puerile' can be similar in meaning to 'immature')
'turn up on ecstasy to provoke anger' is the wordplay.
'turn' shows that the letters should be reversed in order.
'up on' becomes 'up' ('up' can be similar in meaning to 'up on').
'ecstasy' becomes 'e' (informal term for the drug).
'to provoke anger' becomes 'rile'.
'up' reversed gives 'pu'.
(Other definitions for puerile that I've seen before include "Childishly stupid", "Babyish", "Childish, silly, trivial", "Silly, trivial", "Childish, immature", "Infantile" and "Childishly silly".)