First things the key man says? (7,5)

'says?' is the definition.
I know nothing about this answer so I can't tell whether this works.

'first things the key man' is the wordplay.
I cannot really see how this works, but
'first' could be 'opening' (similar in meaning) and 'opening' is present in the answer.
'key' could be 'd' (musical key) and 'd' is located in the answer.
'man' could be 'r' (abbreviation for rook, a chess piece or 'man') and 'r' is present in the answer.
The remaining letters 'wos' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?