Underground part of passage reportedly allowed (7)
I believe the answer is:
rootlet
'underground part' is the definition.
(part of a plant)
'passage reportedly allowed' is the wordplay.
'passage' becomes 'route'.
'reportedly' indicates a 'sounds like' (homophone) clue.
'allowed' becomes 'let' (to let is to allow or permit).
'route' sounds like 'root'.
'root'+'let'='ROOTLET'
'of' is the link.
(Other definitions for rootlet that I've seen before include "vegetation" , "Little plant part" , "Small branch of a plant's "anchor"" , "Small underground part of plant" , "Small rhizome" .)