Pass hotel employee pressing uniform for bookseller (10)

'bookseller' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'pass hotel employee pressing uniform' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'hotel employee' becomes 'porter' (type of hotel employee).
'pressing' indicates putting letters inside.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'porter' enclosing 'u' is 'porteur'.
'col'+'porteur'='COLPORTEUR'

'for' is the link.

Can you help me to learn more?