Some blossoms on climbing plant (4)
I believe the answer is:
moss
'plant' is the definition.
(I've seen this in other clues)
'some blossoms on climbing' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'ssom' is hidden in the letters of 'blossoms on'.
'ssom' written backwards gives 'MOSS'.
(Other definitions for moss that I've seen before include "Damp-loving plant" , "Spore-bearing plant; bog" , "Type of plant growing in damp places" , "Plant growing on many lawns" , "evidence of neglect?" .)