Indignation that’s not in fashion? (7)
I believe the answer is:
outrage
'indignation' is the definition.
('outrage' can be a synonym of 'indignation')
'not in fashion?' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'fashion?' becomes 'rage' (rage is a kind of fashion**).
'out'+'rage'='OUTRAGE'
'that's' is the link.
(Other definitions for outrage that I've seen before include "A feeling of righteous anger" , "Shock grossly" , "Deplorable insult" , "Shocked anger" , "Appalled reaction" .)