Estimation of beasts with about a thousand delivered (10)
I believe the answer is:
assessment
'estimation' is the definition.
(I've seen this before)
'beasts with about a thousand delivered' is the wordplay.
'beasts with' becomes 'asses' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'delivered' becomes 'sent' (I've seen this in another clue).
'm' inserted into 'sent' is 'sment'.
'asses'+'sment'='ASSESSMENT'
'of' is the link.
Can you help me to learn more?
(Other definitions for assessment that I've seen before include "Review" , "Appraisal" , "Opinion" , "test" , "Evaluation, estimation" .)