Something very good seen back in Dudley Zoo days (5)

'something very good' is the definition.
(doozy can mean an excellent example of something)

'back in dudley zoo days' is the wordplay.
'back' says the letters should be written in reverse.
'in' says the answer is hidden in the clue.
'yzood' can be found hidden inside 'dudley zoo days'.
'yzood' back-to-front is 'DOOZY'.

'seen' is the link.