Foreigner holding up as best possible (6)
I believe the answer is:
superb
'best possible' is the definition.
'superb' can be an answer for 'best' (thesaurus). I'm not sure about the 'possible' bit.
'foreigner holding up' is the wordplay.
'foreigner' becomes 'serb' (I can't justify this - if you can you should believe this answer much more).
'holding' is an insertion indicator.
'serb' placed around 'up' is 'SUPERB'.
'as' is the link.
Can you help me to learn more?
(Other definitions for superb that I've seen before include "'Excellent, brilliant (6)'" , "Magnificent, quite splendid" , "'Excellent, splendid (6)'" , "Admirable" , "Tiptop" .)