A mess having tea thus served up (5)
I believe the answer is:
chaos
'a mess' is the definition.
(I've seen this before)
'tea thus served up' is the wordplay.
'tea' becomes 'cha' (cha is a type of tea).
'thus' becomes 'so' (synonyms).
'served up' says the letters should be written backwards (in a down clue, letters go up).
'so' reversed gives 'os'.
'cha'+'os'='CHAOS'
'having' is the link.
(Other definitions for chaos that I've seen before include "A total shambles" , "Pandemonium" , "Anarchy" , "disarray" , "State of extreme confusion and disorder" .)