Composed of extremely powerful and caustic chemical (6)
I believe the answer is:
placid
'composed' is the definition.
(I've seen this before)
'extremely powerful and caustic chemical' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'and' says to put letters next to each other.
'caustic chemical' becomes 'acid' (I am not sure about this - if you are sure you should believe this answer much more).
'powerful' with its middle removed is 'pl'.
'pl'+'acid'='PLACID'
'of' is the link.
Can you help me to learn more?
(Other definitions for placid that I've seen before include "Calm, peaceful" , "Peaceable and not easily disturbed" , "Calm and tranquil" , "Calm and not easily upset" , "Calm of nature" .)