One has to be introduced to good king's benefactor (5)
I believe the answer is:
giver
'benefactor' is the definition.
(giver is a kind of benefactor)
'one has to be introduced to good king's' is the wordplay.
'one has' becomes 'ive' (contraction of I have).
'to be introduced to' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'king' becomes 'r' (r is abbreviation for rex).
'g'+'r'='gr'
'ive' put into 'gr' is 'GIVER'.
(Other definitions for giver that I've seen before include "Bestower" , "presenter" , "Donor" , "Present source" .)