Fighting inside winged creature's cave (6)
I believe the answer is:
beware
'cave' is the definition.
(I've seen this before)
'fighting inside winged creature's' is the wordplay.
'fighting' becomes 'war'.
'inside' is an insertion indicator.
'winged creature' becomes 'bee' (bee is a kind of creature. I am not sure about the 'winged' bit.).
'war' put within 'bee' is 'BEWARE'.
Can you help me to learn more?
(Other definitions for beware that I've seen before include "Be cautious about" , "Take heed" , "Archaic word for look out, mind yourself" , "Archaic warning to watch out, danger ahead" , "Archaic warning to be careful, danger ahead" .)