Belief, revolutionary one, obsessed captain (5)
I believe the answer is:
bahai
'belief' is the definition.
(Asian religion)
'revolutionary one obsessed captain' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'one' becomes 'i' (Roman numeral).
'obsessed captain' becomes 'ahab' (I've seen this in another clue).
'i'+'ahab'='iahab'
'iahab' back-to-front is 'BAHAI'.
(Other definitions for bahai that I've seen before include "Religion of peace/unity" , "Religious follower" , "Religion emphasising unity and peace" , "Religion seeking world peace" , "faith" .)