Tube is beneath reason (11)
I believe the answer is:
underground
'tube' is the definition.
(I know that tube can be written as underground)
'beneath reason' is the wordplay.
'beneath' becomes 'under' (under means beneath or underneath).
'reason' becomes 'ground' (synonyms).
'under'+'ground'='UNDERGROUND'
'is' is the link.
(Other definitions for underground that I've seen before include "Subterranean" , "Secret resistance movement" , "Travel on this" , "Hidden away" , "Tube; covert" .)