Wife joins trendy parliamentary official (4)
I believe the answer is:
whip
'parliamentary official' is the definition.
(whips enforce party discipline in parliament)
'wife joins trendy' is the wordplay.
'wife' becomes 'w' (abbreviation).
'joins' means one lot of letters go next to another.
'trendy' becomes 'hip' (both can mean fashionable).
'w'+'hip'='WHIP'
(Other definitions for whip that I've seen before include "Flagellate" , "The one who enforces discipline in a political party" , "Political officer who ensures party members attend and vote" , "Legislator appointed by a party to enforce discipline" , "Address insultingly" .)