Small measure taken to rein in a cleric (4)
I believe the answer is:
imam
'a cleric' is the definition.
(I've seen this before)
'small measure' is the wordplay.
'small' becomes 'im' (I can't justify this - if you can you should give a lot more credence to this answer).
'measure' becomes 'am' (AM is an example).
'im'+'am'='IMAM'
'taken to rein in' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for imam that I've seen before include "Authority on Islamic doctrine and law" , "Prayer-leader in a mosque" , "Person leading devotions in a mosque" , "Strict religious leader" , "Maim (anag)" .)