Clumsy writer got up in it (5)
I believe the answer is:
inept
'clumsy' is the definition.
('inept' can be a synonym of 'clumsy')
'writer got up in it' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'got up' is a reversal indicator.
'in' means one lot of letters goes inside another.
'pen' reversed gives 'nep'.
'nep' inserted inside 'it' is 'INEPT'.
(Other definitions for inept that I've seen before include "Unsuitable, clumsy" , "rubbish" , "Incompetent and ineffectual" , "Absurd or unskilful" , "Unskilful, awkward" .)