Triumphant expression every second from rash lad (3)
I believe the answer is:
aha
'triumphant expression' is the definition.
'every second from rash lad' is the wordplay.
'every second from' means one should take alternating letters.
'rash'+'lad'='rashlad'
The alternating letters of 'rashlad' are 'AHA'.
(Other definitions for aha that I've seen before include "Expression of surprise, satisfaction or triumph" , "Palindromic exclamation of triumph or surprise" , "Triumphant cry" , "Bingo" , "it expresses surprise" .)