The idea that there's not one available (6)
I believe the answer is:
notion
'the idea' is the definition.
(notion is a kind of idea)
'there's not one available' is the wordplay.
'there's' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'available' becomes 'on' (I've seen this in other clues).
'not' put next to 'i' is 'noti'.
'noti'+'on'='NOTION'
'that' is the link.
(Other definitions for notion that I've seen before include "Idea, concept" , "Vague or fanciful idea" , "Vague idea or opinion" , "Idea, belief" , "Vague understanding" .)