Bet loose pants will be unfashionable (8)
I believe the answer is:
obsolete
'be unfashionable' is the definition.
(obsolete means out of date)
'bet loose pants' is the wordplay.
'pants' is an anagram indicator (UK informal term for 'nonsense' or 'rubbish').
'bet'+'loose'='betloose'
'betloose' is an anagram of 'OBSOLETE'.
'will' is the link.
(Other definitions for obsolete that I've seen before include "Pass" , "Boots eel away, being out of date" , "Discontinued" , "No longer current" , "Obese lot may be out of date" .)