Still lacking place in compound for trip (4)
I believe the answer is:
acid
'compound for trip' is the definition.
'acid' can be an answer for 'compound' (thesaurus). I am not certain of the 'for trip' bit.
'still lacking place' is the wordplay.
'still' becomes 'ay' (archaic term for 'always').
'lacking' means to remove the last letter (the word is incomplete or lacking).
'place' becomes 'cid' (I can't explain this - if you can you should believe this answer much more).
'ay' with its final letter removed is 'a'.
'a'+'cid'='ACID'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "bad rain" , "Bitter substance" , "Vinegary" , "Sarcastic (could be sulphuric?)" , "See 4 Across" .)