Boy from LA devious at first (3)
I believe the answer is:
lad
'boy' is the definition.
(informal term for boy)
'from la devious at first' is the wordplay.
'from' indicates anagramming the letters (from the letters).
'first' says to take the initial letters.
The initial letters of 'la devious at' is 'lda'.
'lda' with letters rearranged gives 'LAD'.
(Other definitions for lad that I've seen before include "Scottish lover" , "Shaver" , "Youngster" , "Boy, young man" , "Stable employee" .)