Tranquil hiding place for illegal substance (4)
I believe the answer is:
acid
'illegal substance' is the definition.
'acid' can be an answer for 'substance' (acid is a kind of substance). I'm not sure about the 'illegal' bit.
'tranquil hiding place' is the wordplay.
'tranquil' becomes 'placid' (I've seen this before).
'hiding' indicates named letters should be taken away.
'place' becomes 'pl' (abbreviation used in postal addresses).
'placid' with 'pl' taken away is 'ACID'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "See 4 Across" , "Acerbic" , "Sarcastic (could be sulphuric?)" , "Sour to the taste" , "Morose" .)