Arrest old boy, one in powerful position (5)
I believe the answer is:
nabob
'one in powerful position' is the definition.
(historical term for a Mughal Empire official)
'arrest old boy' is the wordplay.
'arrest' becomes 'nab' ('nab' can be a synonym of 'arrest').
'old boy' becomes 'ob'.
'nab'+'ob'='NABOB'
(Other definitions for nabob that I've seen before include "very wealthy man" , "Person of conspicuous wealth or high status" , "he may be loaded" , "One divining (water)" , "Magnate; formerly one with Eastern wealth" .)