Ruler shows extremes of devilry, unpleasant no end (6)
I believe the answer is:
dynast
'ruler' is the definition.
(dynast is a kind of ruler)
'extremes of devilry unpleasant no end' is the wordplay.
'extremes of devilry' becomes 'dy' (I can't justify this - if you can you should give a lot more credence to this answer).
'unpleasant' becomes 'nasty' ('nasty' is associated in meaning with 'unpleasant').
'no end' means to remove the last letter.
'nasty' with its final letter removed is 'nast'.
'dy'+'nast'='DYNAST'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for dynast that I've seen before include "Member of a hereditary ruling family" , "Powerful hereditary ruler" , "One ruling" .)