Let down — initially dud is assigned to position (12)
I believe the answer is:
disappointed
'let down' is the definition.
(to let someone down is to disappoint them)
'is assigned to position' is the wordplay.
'is assigned' becomes 'appointed' (similar in meaning).
'to' says to put letters next to each other (I've seen this in other clues).
'position' becomes 'dis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'appointed' after 'dis' is 'DISAPPOINTED'.
'initially dud' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for disappointed that I've seen before include "Unhappy" , "Let-down, frustrated" , "Saddened" , "Let down" , "Feeling sad" .)