Arbutus is not certain to blossom (9)

I believe the answer is:

mayflower

'arbutus' is the definition.
(trailing arbutus is a name for the mayflower)

'is not certain to blossom' is the wordplay.
'is not certain to' becomes 'may'.
'blossom' becomes 'flower' (synonyms).
'may'+'flower'='MAYFLOWER'

(Other definitions for mayflower that I've seen before include "vessel crossing ocean" , "Historic vessel" , "Hawthorn blossom, carrying the ..." , "Pilgrim Fathers' vessel" , "Famous vessel" .)

I've seen this clue in The Guardian.

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