Tea-times revised in approximation (8)
I believe the answer is:
estimate
'approximation' is the definition.
(synonyms)
'tea-times revised' is the wordplay.
'revised' indicates an anagram.
'tea'+'times'='teatimes'
'teatimes' is an anagram of 'ESTIMATE'.
'in' acts as a link.
(Other definitions for estimate that I've seen before include "Rough cost" , "Tease Tim (anag.)" , "Calculate - valuation" , "Statement giving the likely cost of a job" , "Make an educated guess" .)