So my care is to propagate a tree (8)
I believe the answer is:
sycamore
'a tree' is the definition.
(sycamore is a kind of tree)
'so my care is to propagate' is the wordplay.
'is to propagate' indicates an anagram (reordered letters generate or propagate the answer).
'so'+'my'+'care'='somycare'
'somycare' anagrammed gives 'SYCAMORE'.
(Other definitions for sycamore that I've seen before include "Timber producer" , "Mr O'Casey is in the tree" , "Tree, form of maple" , "Tree - so creamy (anag)" , "Tree - may score (anag)" .)