Caution about, again, the underground tunnel (6)
I believe the answer is:
warren
'the underground tunnel' is the definition.
'warren' can be an answer for 'tunnel' (warren is a kind of tunnel). I am not certain of the 'the underground' bit.
'caution about again' is the wordplay.
'caution' becomes 'warn' (I've seen this in another clue).
'about' is an insertion indicator.
'again' becomes 're' (prefix meaning 'again').
'warn' enclosing 're' is 'WARREN'.
Can you help me to learn more?
(Other definitions for warren that I've seen before include "Rabbit home" , "Network of tunnels created by rabbits" , "Series of rabbit burrows" , "Maze of passages" , "Network of underground tunnels built by rabbits" .)