Watch out for flyer having struggle (6)
I believe the answer is:
beware
'watch out' is the definition.
(bewaring is a kind of watching out)
'for flyer having struggle' is the wordplay.
'for flyer' becomes 'bee' (I can't explain this - if you can you should give a lot more credence to this answer).
'having' means one lot of letters goes inside another.
'struggle' becomes 'war' (warring is a kind of struggling).
'bee' placed around 'war' is 'BEWARE'.
Can you help me to learn more?
(Other definitions for beware that I've seen before include "Be on one's guard" , "See 18" , "Have caution, be on guard" , "Archaic warning to watch out, danger ahead" , "Cry of warning" .)