A set of three raised about then (9)
I believe the answer is:
thereupon
'then' is the definition.
(synonyms)
'a set of three raised about' is the wordplay.
'a set of' indicates an anagram (I've seen 'set' mean this (instruction to set the letters)).
'raised' becomes 'up' (similar in meaning).
'about' becomes 'on' (as in a book 'on' a certain topic).
'three' is an anagram of 'there'.
'there'+'up'+'on'='THEREUPON'
(Other definitions for thereupon that I've seen before include "straight afterwards" , "at that point" , "straightaway" , "Shortly after that" , "immediately after that" .)