Her caging a bird is a mistake! (6)
I believe the answer is:
howler
'a mistake' is the definition.
(I have seen 'Serious mistake' mean 'howler' so perhaps 'mistake' could also mean 'howler')
'her caging a bird' is the wordplay.
'caging' is an insertion indicator (I've seen this in other clues).
'a bird' becomes 'owl' (owl is a kind of bird).
'her' placed around 'owl' is 'HOWLER'.
'is' acts as a link.
(Other definitions for howler that I've seen before include "Schoolboy error - wolf?" , "Grievous error" , "Glaring error" , "Stupid glaring mistake" , "Serious avoidable mistake" .)