Spotted president once at welcoming party (5-3)
I believe the answer is:
polka dot
'spotted' is the definition.
(I've seen this before)
'president once at welcoming party' is the wordplay.
'president once' becomes 'polk' (I can't explain this - if you can you should believe this answer much more).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'party' becomes 'do' (do can informally mean a party).
'at' placed around 'do' is 'adot'.
'polk'+'adot'='POLKA-DOT'
Can you help me to learn more?
(Other definitions for polka dot that I've seen before include "Patterned with large spots" , "Design" , "Spotted" , "Kind of fabric with a circular spot pattern (used for teeny-weeny bikinis?)" , "(Of fabric) covered in small, circular regularly-spaced spots" .)