Fox starts to be evasive, catching birds (6)
I believe the answer is:
Here is my best explanation:
'fox' is the definition.
'starts to be evasive catching birds' is the subsidiary indication.
'starts to' says to take the initial letters.
'catching' indicates putting letters inside.
'birds' becomes 'emus' (emu is a kind of bird).
The first letters of 'be evasive' is 'be'.
'be' going around 'emus' is 'bemuse'.
(Other definitions for bemuse that I've seen before include "Fox", "Puzzle and confuse", "Cause puzzlement", "Confound", "Bewilder or baffle", "Confuse, bewilder", "Puzzle, make confused", "Nonplus", "Stupefy, puzzle" and "Perplex".)