Cage up-ended into it, clumsy (5)
I believe the answer is:
inept
'clumsy' is the definition.
(I know that clumsy can be written as inept)
'cage up-ended into it' is the wordplay.
'cage' becomes 'pen' (both can mean an enclosure).
'up-ended' says the letters should be written in reverse (in a down clue, letters go up).
'into' is an insertion indicator.
'pen' written backwards gives 'nep'.
'nep' inserted into 'it' is 'INEPT'.
(Other definitions for inept that I've seen before include "Incompetent and ineffectual" , "Clumsy, bumbling" , "Cack-handed" , "Foolish" , "Clumsy, out of place" .)