Illegal tunnel regularly very poor (8)
I believe the answer is:
unlawful
'illegal' is the definition.
(similar in meaning)
'tunnel regularly very poor' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'very poor' becomes 'awful' (I've seen this before).
The alternate letters of 'tunnel' are 'unl'.
'unl'+'awful'='UNLAWFUL'
(Other definitions for unlawful that I've seen before include "not right" , "Not permitted by the state, contrary to statutes" , "Not legally permitted" , "Prohibited by legislation" , "Illegitimate" .)