Fellow from Riga, maybe, kind of blue (6)
I believe the answer is:
'kind of blue' is the definition.
(I've seen this in another clue)
'fellow from riga maybe' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'from' means one lot of letters go next to another.
'riga maybe' becomes 'balt' (I can't explain this - if you can you should believe this answer much more).
Can you help me to learn more?
(Other definitions for cobalt that I've seen before include "Deep blue pigment" , "Magnetic metal" , "Metal giving its name to type of deep blue" , "Metallic element, Co" , "Silvery-white metal used in alloys" .)