The thing around write-up is unprofessional (5)
I believe the answer is:
inept
'unprofessional' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'the thing around write-up' is the wordplay.
'the thing' becomes 'it'.
'around' is an insertion indicator.
'write' becomes 'pen' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pen' backwards is 'nep'.
'it' placed around 'nep' is 'INEPT'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for inept that I've seen before include "Clumsy, bumbling" , "Clumsy - silly" , "Clumsy and incompetent" , "Futile" , "Unskilful or unsuitable" .)