The branch angle is to attack about ten (4)
I believe the answer is:
axil
'the branch angle' is the definition.
(I've seen this in another clue)
'attack about ten' is the wordplay.
'attack' becomes 'ail' (I can't explain this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'ten' becomes 'X' (Roman numeral).
'ail' enclosing 'x' is 'AXIL'.
'is to' acts as a link.
Can you help me to learn more?
(Another definition for axil that I've seen is " Angle between leaf and stem".)