Sat for claim on the first of December (5)
I believe the answer is:
posed
'sat for' is the definition.
(I've seen this in another clue)
'claim on the first of december' is the wordplay.
'claim' becomes 'pose' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the first of december' becomes 'd' (pst letter of 'december').
'pose'+'d'='POSED'
Can you help me to learn more?
(Other definitions for posed that I've seen before include "Presented (a question)" , "Positioned" , "Struck attitude, perhaps for painter" , "Sat (for artist)" , "Arranged for pictorial purposes" .)