Blue sky sometimes is unavailable (4)
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" .)