Follow key number, heading off for battle (8)
I believe the answer is:
dogfight
'off for battle' is the definition.
'dogfight' can be an answer for 'battle' (thesaurus). I'm not sure about the 'off for' bit.
'follow key number heading' is the wordplay.
'follow' becomes 'dog' (I've seen this before).
'key number heading' becomes 'fight' (I can't justify this - if you can you should believe this answer much more).
'dog'+'fight'='DOGFIGHT'
Can you help me to learn more?
(Other definitions for dogfight that I've seen before include "Canine conflict or combat between fighter planes" , "Highly aggressive activity" , "Aircraft battle" , "Mle" , "Close combat between fighter aircraft" .)