Tunnel Abe engineered, impossible to defend (9)
I believe the answer is:
untenable
'impossible to defend' is the definition.
(something untenable cannot be defended or excused)
'tunnel abe engineered' is the wordplay.
'engineered' indicates an anagram (I've seen 'engineer' mean this).
'tunnel'+'abe'='tunnelabe'
'tunnelabe' is an anagram of 'UNTENABLE'.
(Other definitions for untenable that I've seen before include "Indefensible" , "unworthy of support?" , "Not defensible" , "Incapable of being maintained or defended" , "impossible to support" .)