Evaluate dunces, repeating one part for the fourth time (6)
I believe the answer is:
assess
'evaluate dunces repeating' is the definition.
'assess' can be an answer for 'evaluate' ('assess' can be a synonym of 'evaluate'). I am unsure of the 'dunces repeating' bit.
'one part for the fourth time' is the wordplay.
'one part' becomes 'ass' (ass is a kind of part).
'for' says to put letters next to each other.
'the fourth time' becomes 'ess' (I can't justify this - if you can you should give a lot more credence to this answer).
'ass'+'ess'='ASSESS'
(Other definitions for assess that I've seen before include "Evaluate, estimate" , "Estimate, gauge" , "Judge the quality of" , "Test" , "Reckon value" .)