China supporting Estonia, I guess (8)
I believe the answer is:
estimate
'guess' is the definition.
(I know that guess can be written as estimate)
'china supporting estonia i' is the wordplay.
'china' becomes 'mate' (china plate is Cockney rhyming slang for mate).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'estonia' becomes 'est' (abbreviation).
'est'+'i'='esti'
'mate' put after 'esti' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Guess, rough calculation" , "rough judgment" , "Statement giving the likely cost of a job" , "Judge to be probable" , "Believe" .)