Guess I'm in the class (8)
I believe the answer is:
estimate
'guess' is the definition.
(I know that guess can be written as estimate)
'i'm in the class' is the wordplay.
'in the' indicates putting letters inside.
'class' becomes 'estate' (estate is a kind of class).
'im' put within 'estate' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Make an approximate assessment" , "Quotation" , "Guess, rough calculation" , "Judge the approximate cost of" , "Guess the cost" .)