Fifty-one competent and responsible (6)
I believe the answer is:
liable
'responsible' is the definition.
(similar in meaning)
'fifty-one competent' is the wordplay.
'fifty one' becomes 'li' (Roman numerals).
'competent' becomes 'able' (similar in meaning).
'li'+'able'='LIABLE'
'and' acts as a link.
(Other definitions for liable that I've seen before include "To blame" , "Legally obligated" , "Apt to" , "'Responsible, answerable (6)'" , "legally obliged, held responsible" .)