Reckon site upset friend (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'site upset friend' is the wordplay.
'upset' is an anagram indicator.
'friend' becomes 'mate' (mate is a kind of friend**).
'site' is an anagram of 'esti'.
'esti'+'mate'='ESTIMATE'
(Other definitions for estimate that I've seen before include "Guess, rough calculation" , "Tease Tim (anag.)" , "Statement of expected cost" , "Evaluate - approximation" , "Make an approximate assessment" .)