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

'says?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'first things the key man' is the wordplay.
I cannot quite understand 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 found within the answer.
'man' could be 'r' (abbreviation for rook, a chess piece or 'man') and 'r' is found in the answer.
The remaining letters 'wos' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?