Arab ruler takes part in rebellious experiment (4)
I believe the answer is:
emir
'arab ruler' is the definition.
(I've seen this before)
'part in rebellious experiment' is the wordplay.
'rebellious' says the letters should be written in reverse.
'part in' says the answer is hidden in the clue.
'rime' can be found hidden inside 'experiment'.
'rime' back-to-front is 'EMIR'.
'takes' is the link.
(Other definitions for emir that I've seen before include "leader in the East" , "Eastern ruler in mire" , "eader abroad?" , "Title of various rulers; I'm ER (anag.)" , "Top man" .)