Dark dirty place, adult in spirit (7)
I believe the answer is:
stygian
'dark' is the definition.
(similar in meaning)
'dirty place adult in spirit' is the wordplay.
'dirty place' becomes 'sty' (I've seen this in another clue).
'adult' becomes 'a' (abbreviation).
'in' indicates putting letters inside.
'spirit' becomes 'gin' (alcoholic spirit).
'a' inserted within 'gin' is 'gian'.
'sty'+'gian'='STYGIAN'
(Other definitions for stygian that I've seen before include "Gloomy or murky after the river Styx" , "Very dark (poetic)" , "Dark and gloomy, like the mythological river Styx" , "underworld runner" .)