Clumsy, I knock over writer with front of trolley (5)
I believe the answer is:
inept
'clumsy' is the definition.
('inept' can be a synonym of 'clumsy')
'i knock over writer with front of trolley' is the wordplay.
'knock over' shows that the letters should be reversed in order.
'writer with' becomes 'pen' (I've seen this in other clues).
'front of' says to take the initial letters.
The first letter of 'trolley' is 't'.
'pen' reversed gives 'nep'.
'i'+'nep'+'t'='INEPT'
(Other definitions for inept that I've seen before include "Unskilful or unsuitable" , "Maladroit" , "Absurd or unskilful" , "Incompetent and ineffectual" , "Useless, awkward" .)