Her caging a bird is a mistake! (6)
I believe the answer is:
howler
'is a mistake' is the definition.
(I know that howler is a type of boner)
'her caging a bird' is the wordplay.
'caging' means one lot of letters goes inside another (I've seen this in other clues).
'a bird' becomes 'owl' (owl is a kind of bird).
'her' enclosing 'owl' is 'HOWLER'.
(Other definitions for howler that I've seen before include "Schoolboy error - wolf?" , "Grievous error" , "Glaring error" , "Stupid glaring mistake" , "Serious avoidable mistake" .)