Guess site upset friend (8)
I believe the answer is:
estimate
'guess' is the definition.
(I know that guess can be written as estimate)
'site upset friend' is the wordplay.
'upset' indicates an anagram.
'friend' becomes 'mate' (mate is a kind of friend).
'site' with letters rearranged gives 'esti'.
'esti'+'mate'='ESTIMATE'
(Other definitions for estimate that I've seen before include "Judge the approximate value" , "Believe" , "Form opinion of - tea times (anag)" , "Gauge - approximation" , "Guess, rough calculation" .)