To lacerate is frightening, if poking inside (7)
I believe the answer is:
scarify
'to lacerate' is the definition.
(thesaurus)
'frightening if poking inside' is the wordplay.
'frightening' becomes 'scary' (I've seen this before).
'poking inside' means one lot of letters goes inside another (I've seen this in other clues).
'scary' placed around 'if' is 'SCARIFY'.
'is' acts as a link.
(Other definitions for scarify that I've seen before include "Break up the surface (of soil)" , "Scratch the surface" , "Cut and remove debris from (a lawn)" , "Severely criticise" .)