Evaluate last of loads carried by beasts of burden (6)
I believe the answer is:
assess
'evaluate' is the definition.
('assess' can be a synonym of 'evaluate')
'last of loads carried by beasts of burden' is the wordplay.
'last of' says to take the final letters.
'carried by' indicates putting letters inside.
'beasts of burden' becomes 'asses' (I've seen this before).
The final letter of 'loads' is 's'.
's' inserted within 'asses' is 'ASSESS'.
(Other definitions for assess that I've seen before include "'Judge, estimate (6)'" , "Estimate value (for tax)" , "(Of a tax) determine the amount to be paid" , "Judge the quality of" , "Rate, evaluate" .)