Take a look into a large juvenile compound (8)
I believe the answer is:
alkaloid
'compound' is the definition.
(alkaloid is a kind of compound)
'take a look into a large juvenile' is the wordplay.
'take a look' becomes 'alo' (I am not sure about this - if you are sure you should believe this answer much more).
'into' means one lot of letters goes inside another.
'large' becomes 'l'.
'juvenile' becomes 'kid' (kid is a kind of juvenile).
'a'+'l'+'kid'='alkid'
'alo' placed inside 'alkid' is 'ALKALOID'.
(Other definitions for alkaloid that I've seen before include "Organic compound with toxic or medicinal property" , "Plant compound having physiological effect on humans" , "Organic compound having physiological action" , "Any of a group of nitrogenous basic compounds found in plants" .)