500 against adult being bitten by naughty canine earlier (2,7)
I believe the answer is:
in advance
'earlier' is the definition.
(at an earlier point in time)
'500 against adult being bitten by naughty canine' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'against' becomes 'v' (abbreviation for 'versus').
'adult being' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'bitten by' indicates putting letters inside (I've seen this in other clues).
'naughty' is an anagram indicator.
'd'+'v'+'a'='dva'
'canine' anagrammed gives 'inance'.
'dva' placed inside 'inance' is 'IN ADVANCE'.
Can you help me to learn more?
(Other definitions for in advance that I've seen before include "Before the event" , "Ahead in place or time" , "Beforehand - can invade accordingly (2.7)" , "early" .)