Show American city always mounts (6)
I believe the answer is:
reveal
'show' is the definition.
(I know that reveal is a more specific form of the action show)
'american city always mounts' is the wordplay.
'american city' becomes 'la' (Los Angeles).
'always' becomes 'ever' (synonyms).
'mounts' shows that the letters should be reversed in order (I've seen 'mounting' mean this (going up in a down clue)).
'la'+'ever'='laever'
'laever' written backwards gives 'REVEAL'.
(Other definitions for reveal that I've seen before include "Remove cover, unveil" , "Make known, uncover and allow to be seen" , "Make known or visible" , "'Make known, disclose (6)'" , "let out" .)