Nobleman set about restraining unruly cousin (8)
I believe the answer is:
viscount
'nobleman' is the definition.
(I know that viscount is a type of nobleman)
'set about restraining unruly cousin' is the wordplay.
'set' becomes 'tv' (TV set).
'about' says the letters should be written in reverse.
'restraining' indicates putting letters inside.
'unruly' is an anagram indicator.
'tv' reversed gives 'vt'.
'cousin' with letters rearranged gives 'iscoun'.
'vt' placed around 'iscoun' is 'VISCOUNT'.
(Other definitions for viscount that I've seen before include "Lord" , "plane" , "British noble" , "Minor peer" , "Nobleman" .)