Tall cavalry displacing small dockers (12)
I believe the answer is:
longshoremen
'dockers' is the definition.
('longshoreman' can be a synonym of 'docker')
'tall cavalry' is the wordplay.
'tall' becomes 'long' (I've seen this before).
'cavalry' becomes 'shoremen' (I am not sure about this - if you are sure you should believe this answer much more).
'long'+'shoremen'='LONGSHOREMEN'
'displacing small' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Another definition for longshoremen that I've seen is " US dockers".)