Serving special drink to welcome king (7)
I believe the answer is:
portion
'serving special' is the definition.
'portion' can be an answer for 'serving' (I've seen this in another clue). I'm not sure about the 'special' bit.
'drink to welcome king' is the wordplay.
'drink' becomes 'port' (port is a kind of drink).
'to welcome king' becomes 'ion' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'port'+'ion'='PORTION'
Can you help me to learn more?
(Other definitions for portion that I've seen before include "Part - share" , "Part of a whole" , "Amount of food" , "A lot of" , "Helping - fate" .)