Boast erroneously about a flower (5)
I believe the answer is:
aster
'a flower' is the definition.
(type of flower)
'boast erroneously about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'ASTER' is hidden within 'boast erroneously'.
(Other definitions for aster that I've seen before include "Stare oddly at the flower" , "Stare (anag.)" , "Stare at pretty flower" , "Large daisy" , "(Starlike) flower" .)