The sole doubt relates to being unavailable (4,3)
I believe the answer is:
sold out
'the sole doubt relates' is the definition.
I can't judge whether this definition defines the answer.
'being unavailable' is the wordplay.
'being' becomes 'sold' (I can't explain this - if you can you should believe this answer much more).
'unavailable' becomes 'out' (I've seen this in another clue).
'sold'+'out'='SOLD-OUT'
'to' is the link.
Can you help me to learn more?
(Another definition for sold out that I've seen is " None left in the shop".)