Cat, one stuck in tree in compound (8)
I believe the answer is:
ptomaine
'compound' is the definition.
(ptomaine is a kind of compound)
'cat one stuck in tree' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'one' becomes 'a'.
'stuck in' is an insertion indicator.
'tree' becomes 'pine' (pine is a kind of tree**).
'tom'+'a'='toma'
'toma' inserted within 'pine' is 'PTOMAINE'.
'in' is the link.
(Other definitions for ptomaine that I've seen before include "stuff of death?" , "Poisoning, for instance" , "Food poisoning from rotten substances" , "possible source of poisoning" , "Sort of poisoning" .)