Youth from LA demure at first (3)
I believe the answer is:
lad
'youth' is the definition.
(I've seen this in other clues)
'from la demure at first' is the wordplay.
'from' indicates an anagram (from the letters).
'first' says to take the initial letters.
The initial letters of 'la demure at' is 'lda'.
'lda' with letters rearranged gives 'LAD'.
(Other definitions for lad that I've seen before include "Youngster" , "Shaver" , "Young fellow" , "Chap, fellow" , "Boy, young man" .)