Small measure to rein in a cleric (4)
I believe the answer is:
imam
'cleric' is the definition.
(I've seen this before)
'small measure to rein in a' is the wordplay.
'small measure' becomes 'm' (m is a kind of measure; 'small' suggests an abbreviation).
'to' says to put letters next to each other (I've seen this in other clues).
'rein in' becomes 'im' (I can't justify this - if you can you should believe this answer much more).
'im'+'a'='ima'
'm' after 'ima' is 'IMAM'.
Can you help me to learn more?
(Other definitions for imam that I've seen before include "Maim (anag)" , "Mosque's prayer leader" , "Islamic prayer leader" , "Person leading devotions in a mosque" , "Muslim community leader" .)