Worker in spotty shirt mistaken for secretary (9,6)

'secretary' is the definition.
(shorthand typist is a kind of secretary)

'worker in spotty shirt mistaken' is the wordplay.
I cannot quite understand how this works, but
'worker' could be 'typist' (typist is a kind of worker) and 'typist' is found within the answer.
'shirt' could be 't' (as in a T-shirt) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'for' acts as a link.

Can you help me to learn more?