Sight rarely seen on British beaches nowadays depressed Jack (4,4)

I believe the answer is:

blue flag

I'm a little stuck... Click here to teach me more about this clue!

'sight rarely seen on british beaches nowadays' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'depressed jack' is the wordplay.
'depressed' becomes 'blue' (I've seen this in another clue).
'jack' becomes 'flag' (I've seen this before).
'blue'+'flag'='BLUE FLAG'

Can you help me to learn more?

(Another definition for blue flag that I've seen is " Pennant awarded to clean beaches".)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app