Crossword Genius

Blue sky sometimes is unavailable (4)

Ross

I believe the answer is:

busy

'unavailable' is the definition.

'blue sky sometimes' is the wordplay.
'sometimes' means one should take alternating letters (some of the letters are taken at regular intervals).
'blue'+'sky'='bluesky'
The alternating letters of 'bluesky' are 'BUSY'.

'is' is the link.

(Other definitions for busy that I've seen before include "Active (as a bee?)" , "Fully employed" , "Like a bee?" , "Active, occupied" , "With much to do" .)

I've seen this clue in The Independent.
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