Set up and clued up about mess up (8)
I believe the answer is:
assemble
'set up' is the definition.
(synonyms)
'clued up about mess up' is the wordplay.
'clued up' becomes 'able' (both can mean knowledgeable or competent).
'about' indicates putting letters inside.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'mess' in reverse letter order is 'ssem'.
'able' going around 'ssem' is 'ASSEMBLE'.
'and' acts as a link.
(Other definitions for assemble that I've seen before include "Gather or collect together" , "Bring/come together" , "Bring or gather together" , "Gather together in one place" , "Build" .)