Jokey labels stuck behind a ship when keeping watch? (6,8)

'ship when keeping watch?' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'jokey labels stuck behind a' is the wordplay.
'jokey' becomes 'bum' (I can't explain this - if you can you should believe this answer much more).
'labels' becomes 'stickers' ('sticker' can be a synonym of 'label').
'stuck behind' means one lot of letters go next to another (I've seen this in other clues).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'stickers' put after 'per' is 'perstickers'.
'bum'+'perstickers'='BUMPER STICKERS'

Can you help me to learn more?