Young and old initially light up Alpine air? (5)
I believe the answer is:
yodel
'alpine air?' is the definition.
(I've seen this in another clue)
'young and old initially light up' is the wordplay.
'young' becomes 'y'.
'and' says to put letters next to each other.
'initially' suggests taking the first letters.
'light' becomes 'led' (light-emitting diode).
'up' is a reversal indicator (in down clue: letters go upwards).
The first letter of 'old' is 'o'.
'led' back-to-front is 'del'.
'y'+'o'+'del'='YODEL'
(Other definitions for yodel that I've seen before include "Sing by alternating between normal voice and falsetto" , "Sing partly falsetto" , "Alpine warble" , "Warbling voice" , "Sing in intermittent falsetto" .)