Captain of warship maybe securing job as mail worker (3-10)

'mail worker' is the definition.
(someone working with mail or post)

'captain of warship maybe securing job' is the wordplay.
I cannot really understand how this works, but
'captain' could be 'master' (synonyms) and 'master' is found in the answer.
'of' could be 'o' and 'o' is located in the answer.
'warship' could be 'sub' (short for submarine, a submersible warship) and 'sub' is located in the answer.
The remaining letters 'pst' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).

'as' acts as a link.