Weigh blockheads on top of scales (6)
I believe the answer is:
assess
'weigh' is the definition.
(I've seen this before)
'blockheads on top of scales' is the wordplay.
'blockheads' becomes 'asses' (I've seen this in another clue).
'on top of' indicates taking the first letters.
The first letter of 'scales' is 's'.
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Put a value on" , "Evaluate worth" , "Estimate, decide the value of" , "Evaluate, estimate" , "Place a value on" .)