Frighten in heartless manner with cunning intervention (7)
I believe the answer is:
startle
'frighten' is the definition.
(thesaurus)
'in heartless manner with cunning intervention' is the wordplay.
'in' is an insertion indicator.
'heartless' suggests removing the centre.
'manner' becomes 'style' ('style' can be a synonym of 'manner').
'with cunning intervention' becomes 'art' (I am not sure about this - if you are sure you should believe this answer much more).
'style' with its middle taken out is 'stle'.
'stle' going around 'art' is 'STARTLE'.
(Other definitions for startle that I've seen before include "Surprise with starlet" , "Cause to jump" , "Take unawares with rattles" , "Surprise with fright" , "Alarm (someone)" .)