King takes on credit for nobleman (4)
I believe the answer is:
duke
'for nobleman' is the definition.
(duke is a kind of nobleman)
'king takes on credit' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'takes on' indicates putting letters inside (inserted letters are worn or taken on).
'credit' becomes 'due' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'k' inserted into 'due' is 'DUKE'.
Can you help me to learn more?
(Other definitions for duke that I've seen before include "Fist (sl.)" , "Highest-ranking nobleman" , "Lord" , "High hereditary title in Britain" , "noble" .)