Surprised man on alert for decorative plant (10)
I believe the answer is:
ornamental
'decorative plant' is the definition.
(I know that ornamental is a type of decorative plant)
'surprised man on alert' is the wordplay.
'surprised' indicates an anagram.
'man'+'on'+'alert'='manonalert'
'manonalert' anagrammed gives 'ORNAMENTAL'.
'for' is the link.
(Other definitions for ornamental that I've seen before include "Decorative" , "showy" , "non-functional" , "Fancy" , "Plant grown for attractive appearance" .)