Take appropriate external remedy (7)
I believe the answer is:
capture
'take' is the definition.
(capturing is a kind of taking)
'appropriate external remedy' is the wordplay.
'appropriate' becomes 'apt' ('apt' can be a synonym of 'appropriate').
'external' is an insertion indicator (I've seen this in other clues).
'remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
'apt' put within 'cure' is 'CAPTURE'.
(Other definitions for capture that I've seen before include "Take by force" , "Take prisoner" , "Apprehend" , "Ensnare" , "Take (eg, chess piece)" .)