It's brought back in The Return of the King , perhaps - must be radical (7)
I believe the answer is:
'radical' is the definition.
(I've seen this before)
'it's brought back in the return of the king perhaps' is the wordplay.
'brought back' is a reversal indicator.
'in the' means one lot of letters goes inside another.
'return of' says the letters should be written in reverse.
'the king perhaps' becomes 'card' (king is a kind of card).
'its' written backwards gives 'sti'.
'card' back-to-front is 'drac'.
'sti' inserted within 'drac' is 'DRASTIC'.
'must be' is the link.
(Other definitions for drastic that I've seen before include "Uncompromising" , "Unsparing" , "Draconian" , "Radical, brutal" , "Extreme - forceful" .)