Fancy sequin soon becoming a must (4,3,3)
I believe the answer is:
sine qua non
'a must' is the definition.
(I've seen this before)
'fancy sequin soon' is the wordplay.
'fancy' indicates an anagram.
'soon' becomes 'anon' (synonyms).
'sequin' with letters rearranged gives 'sinequ'.
'sinequ'+'anon'='SINE QUA NON'
'becoming' acts as a link.
(Other definitions for sine qua non that I've seen before include "A vital qualification" , "Must" , "Indispensable condition" , "Essential condition" .)