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 am unsure of 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 believe this answer much more).
'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" .)