He rules fifth-rate politician’s a gutless mistake (7)
I believe the answer is:
emperor
'he rules' is the definition.
(type of ruler)
'fifth-rate politician's a gutless mistake' is the wordplay.
'fifth-rate' becomes 'e' (fifth worst category).
'politician' becomes 'mp' (Member of Parliament).
'a gutless' means to remove the middle letters.
'mistake' becomes 'error' (synonyms).
'error' with its middle taken out is 'eror'.
'e'+'mp'+'eror'='EMPEROR'
(Other definitions for emperor that I've seen before include "Supreme ruler" , "Sovereign and richly-colored butterfly" , "Beethoven piano concerto, Op. 73" , "Largest type of penguin" , "Penguin of largest known species - a great ruler" .)