Lively firework, initially dangerous (6)
I believe the answer is:
frisky
'lively' is the definition.
(I've seen this before)
'firework initially dangerous' is the wordplay.
'initially' says to take the initial letters.
'dangerous' becomes 'risky' ('risky' can be similar in meaning to 'dangerous').
The initial letter of 'firework' is 'f'.
'f'+'risky'='FRISKY'
(Other definitions for frisky that I've seen before include "Energetically playful" , "Lively like playful dog" , "Animated" , "In high spirits" , "High-spirited" .)