Judge Conservatives, ultimately, to be led by donkeys (6)
I believe the answer is:
assess
'judge' is the definition.
(assessing is a kind of judging)
'conservatives ultimately to be led by donkeys' is the wordplay.
'ultimately' indicates one should take the final letters.
'to be led by' is a charade indicator (letters next to each other).
'donkeys' becomes 'asses' (an ass is a donkey).
The final letter of 'conservatives' is 's'.
's' after 'asses' is 'ASSESS'.
(Other definitions for assess that I've seen before include "'Judge, estimate (6)'" , "Remedy for all ills" , "Appraise, estimate value" , "price" , "Judge the quality of" .)