All oak? I'd planned organic compound (8)
I believe the answer is:
alkaloid
'organic compound' is the definition.
(alkaloid is a kind of organic compound)
'all oak? i'd planned' is the wordplay.
'planned' is an anagram indicator.
'all'+'oak'+'id'='alloakid'
'alloakid' anagrammed gives 'ALKALOID'.
(Other definitions for alkaloid that I've seen before include "Plant compound having physiological effect on humans" , "Organic compound having physiological action" , "Any of a group of nitrogenous basic compounds found in plants" , "Organic compound with toxic or medicinal property" .)