Friend supporting Spain set to be gutted, I guess (8)
I believe the answer is:
estimate
'guess' is the definition.
('estimate' can be a synonym of 'guess')
'friend supporting spain set to be gutted i' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend**).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'spain' becomes 'e' (abbreviation for Espana).
'set to' becomes 'set' ('set' can be a synonym of 'set to').
'be gutted' means to remove the middle letters (to gut something is to remove its insides).
'set' with its centre taken out is 'st'.
'e'+'st'+'i'='esti'
'mate' after 'esti' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Judge to be probable" , "Guess, rough calculation" , "Calculate - valuation" , "Likely amount" , "Evaluate - approximation" .)