Clumsy half-nephew breaking that object (5)
I believe the answer is:
inept
'clumsy' is the definition.
('inept' can be a synonym of 'clumsy')
'half-nephew breaking that object' is the wordplay.
'half' indicates taking half.
'breaking' means one lot of letters goes inside another.
'that object' becomes 'it' (pronoun for an inanimate object).
'nephew' halved is 'nep'.
'nep' going inside 'it' is 'INEPT'.
(Other definitions for inept that I've seen before include "Unskilful or unsuitable" , "Clumsy and incompetent" , "Clumsy, out of place" , "Incompetent and ineffectual" , "Clumsy, bumbling" .)