Underground room I see said to be difficult to understand (7)
I believe the answer is:
cryptic
'be difficult to understand' is the definition.
(as in a cryptic message)
'underground room i see said' is the wordplay.
'underground room' becomes 'crypt' (I've seen this before).
'said' shows a homophone (sound like).
'see' sounds like 'c'.
'crypt'+'i'+'c'='CRYPTIC'
'to' acts as a link.
(Other definitions for cryptic that I've seen before include "Like our other puzzle" , "(Of crossword) withindirect clues" , "Esoteric" , "Mysteriously obscure" , "Enigmatic" .)