Three-card trick on film is capital (10)
I believe the answer is:
montevideo
'is capital' is the definition.
(I've seen this before)
'three-card trick on film' is the wordplay.
'three card trick' becomes 'monte' (I can't justify this - if you can 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).
'film' becomes 'video' (I've seen this before).
'monte'+'video'='MONTEVIDEO'
Can you help me to learn more?
(Other definitions for montevideo that I've seen before include "Port" , "Uruguayan capital" , "Capital of Uruguay" .)