Cut flowers providing setting for start of anniversary at twelve o'clock (4,5)

'at twelve o'clock' is the definition.

'cut flowers providing setting for start of anniversary' is the wordplay.
'cut flowers' becomes 'deadhead'.
'providing setting for' means one lot of letters goes inside another (one word is set inside another).
'start of' suggests taking the first letters.
The initial letter of 'anniversary' is 'a'.
'deadhead' enclosing 'a' is 'DEAD AHEAD'.