Symbolic device used in regressive viral video to impress brutal extremists (6)
I believe the answer is:
emblem
'symbolic device' is the definition.
(I've seen this before)
'regressive viral video to impress brutal extremists' is the wordplay.
'regressive' is a reversal indicator (to regress is to go backwards).
'viral video' becomes 'meme' (as in an internet meme).
'to impress' is an insertion indicator (impress can mean enlist or recruit).
'extremists' suggests removing the centre (letters on the extreme ends of the word).
'brutal' with its centre removed is 'bl'.
'meme' backwards is 'emem'.
'emem' placed around 'bl' is 'EMBLEM'.
'used in' is the link.
(Other definitions for emblem that I've seen before include "Visible symbol of an abstract idea" , "Device, badge or symbol" , "Mark" , "Representative object" , "Badge, token" .)