To enter up first class return to paradise (6)
I believe the answer is:
utopia
'paradise' is the definition.
(I've seen this before)
'to enter up first class return' is the wordplay.
'enter' indicates putting letters inside.
'first class' becomes 'ai' (A1 - top quality).
'return' shows that the letters should be reversed in order.
'ai' in reverse letter order is 'ia'.
'to' going within 'up' is 'utop'.
'utop'+'ia'='UTOPIA'
'to' is the link.
(Other definitions for utopia that I've seen before include "Eden" , "Sir Thomas Moore's perfect society" , "Thomas More's ideal state" , "Supposed ideal land" , "More's ideal world" .)