Failing to start sleeping, being clumsy (9)
I believe the answer is:
lumbering
'clumsy' is the definition.
(thesaurus)
'failing to start sleeping' is the wordplay.
'failing to start' means to remove the first letter.
'sleeping' becomes 'slumbering' ('slumber' can be a synonym of 'sleep').
'slumbering' with its initial letter taken away is 'LUMBERING'.
'being' is the link.
(Other definitions for lumbering that I've seen before include "Awkward" , "Ponderous" , "Moving in heavy clumsy way" .)