In retreat a holy man leads king and emperor (4)
I believe the answer is:
tsar
'emperor' is the definition.
(I've seen this before)
'in retreat a holy man leads king' is the wordplay.
'in retreat' shows that the letters should be reversed in order.
'holy man' becomes 'st'.
'leads' means one lot of letters go next to another.
'king' becomes 'r' (r is abbreviation for rex).
'a'+'st'='ast'
'ast' backwards is 'tsa'.
'tsa'+'r'='TSAR'
'and' is the link.
(Other definitions for tsar that I've seen before include "Star for a Russian emperor" , "Word derived from Caesar, used by European rulers since 1547" , "Title given to the kings of Bulgaria" , "Powerful man" , "Formal rulings on Islamic law; was aft (anag.)" .)