American colourist upset East European periodically (2,9)

'periodically' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'american colourist upset east european' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'colourist' becomes 'tinter' (I can't explain this - if you can you should believe this answer much more).
'upset' shows that the letters should be reversed in order.
'east european' becomes 'slav' (the Slavs are a European people).
'slav' backwards is 'vals'.
'a'+'tinter'+'vals'='AT INTERVALS'

Can you help me to learn more?