US docker is eager to meet rider leaving sierra (12)

I believe the answer is:

longshoreman

I'm a little stuck... Click here to teach me more about this clue!

'us docker' is the definition.
'longshoreman' can be an answer for 'docker' (synonyms). I am not certain of the 'us' bit.

'is eager to meet rider leaving sierra' is the wordplay.
'is eager' becomes 'longs' (I can't justify this - if you can you should give a lot more credence to this answer).
'to meet' means one lot of letters go next to another.
'rider' becomes 'horseman' (someone riding a horse).
'leaving' is a deletion indicator.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'horseman' with 's' taken away is 'horeman'.
'longs'+'horeman'='LONGSHOREMAN'

Can you help me to learn more?

(Other definitions for longshoreman that I've seen before include "Stevedore" , "Dock worker" , "docker" .)

I've seen this clue in The Times.

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