Hawthorn, say, is not certain to blossom (9)
I believe the answer is:
mayflower
'hawthorn' is the definition.
The answer and definition can be both plants as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'is not certain to blossom' is the wordplay.
'is not certain to' becomes 'may'.
'blossom' becomes 'flower' (synonyms).
'may'+'flower'='MAYFLOWER'
'say' is the link.
Can you help me to learn more?
(Other definitions for mayflower that I've seen before include "Pilgrim Fathers' ship to the US" , "emigrants' vessel?" , "Tree" , "Hawthorn blossom, carrying the ..." , "Hawthorne mentioned" .)