Watch out for flyer having struggle (6)
I believe the answer is:
beware
'watch out for' is the definition.
(I've seen this before)
'flyer having struggle' is the wordplay.
'flyer' becomes 'bee' (I am not sure about this - if you are sure you should believe this answer much more).
'having' indicates putting letters inside.
'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" .)