The combination could be, all round, very, very dangerous (6)
I believe the answer is:
lethal
'very very dangerous' is the definition.
I can't tell whether this defines the answer.
'the combination could be all round' is the wordplay.
'combination' indicates an anagram.
'could be' indicates anagramming the letters.
'round' is an insertion indicator.
'all' with letters rearranged gives 'lal'.
'the' with letters rearranged gives 'eth'.
'eth' going within 'lal' is 'LETHAL'.
Can you help me to learn more?
(Other definitions for lethal that I've seen before include "Deadly dangerous" , "Killing" , "Noxious" , "bringing down the curtain" , "Enough to cause death" .)