Prayer leader? I am a medium (4)
I believe the answer is:
imam
'prayer leader?' is the definition.
(I've seen this before)
'i am a medium' is the wordplay.
'i am' becomes 'im' (**).
'medium' becomes 'm' (as in UK clothing sizes).
'im'+'a'+'m'='IMAM'
(Other definitions for imam that I've seen before include "Prayer leader in a mosque" , "Maim (anag)" , "Islamic minister" , "Prayer-leader in a mosque" , "Man who leads prayers in a mosque" .)