Unreasonably startle a young actress (7)
I believe the answer is:
starlet
'a young actress' is the definition.
(synonyms)
'unreasonably startle' is the wordplay.
'unreasonably' indicates anagramming the letters.
'startle' with letters rearranged gives 'STARLET'.
(Other definitions for starlet that I've seen before include "young celebrity" , "Young film actress with a future?" , "Promising young film actress" .)