Lively young person scared Dawn at first (6,7)
I believe the answer is:
spring chicken
'lively young person' is the definition.
The definition suggests a singular noun which matches the answer.
'scared dawn at first' is the wordplay.
'scared' becomes 'chicken'.
'dawn' becomes 'spring' (I am not sure about this - if you are sure you should believe this answer much more).
'at first' is a charade indicator (letters next to each other).
'chicken' put after 'spring' is 'SPRING CHICKEN'.
Can you help me to learn more?
(Other definitions for spring chicken that I've seen before include "Young (bird or) person" , "Youngster" , "Young bird (used as disparaging comparison with older people)" , "young one" , "Fowl - type of youth" .)