Fabulous paradise bird held in irrational belief (12)
I believe the answer is:
superstition
'irrational belief' is the definition.
(I've seen this before)
'fabulous paradise bird held' is the wordplay.
'fabulous' becomes 'super'.
'paradise' becomes 'sion' (Hebrew term for paradise).
'bird' becomes 'tit' (tit is a kind of bird).
'held' means one lot of letters goes inside another.
'sion' enclosing 'tit' is 'stition'.
'super'+'stition'='SUPERSTITION'
'in' is the link.
(Other definitions for superstition that I've seen before include "Irrational belief in the supernatural" , "Credulous belief" .)