Prohibit working peer (5)
I believe the answer is:
baron
'peer' is the definition.
(baron is a kind of peer)
'prohibit working' is the wordplay.
'prohibit' becomes 'bar' (to bar is to ban or prohibit).
'working' becomes 'on' (on can mean working or activated).
'bar'+'on'='BARON'
(Other definitions for baron that I've seen before include "Member of the nobility" , "Aristocratic title" , "Nobleman providing a portion of beef" , "A lord; sounds like childless" , "Born a (anagram) - powerful man of business" .)