Relative's very new and fashionable act (3-2-3)
I believe the answer is:
son-in-law
'relative's' is the definition.
(son-in-law is a kind of relative)
'very new and fashionable act' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'and' means one lot of letters go next to another.
'fashionable' becomes 'in' (similar in meaning).
'act' becomes 'law' (law is a kind of act).
'so'+'n'+'in'+'law'='SON-IN-LAW'
(Other definitions for son-in-law that I've seen before include "Relative by marriage" , "Daughter's husband" , "Member of extended family" , "He's married to your daughter" .)