Knock out the head of security with a tool (4)
I believe the answer is:
'tool' is the definition.
(rasp is a kind of tool)
'knock out the head of security' is the wordplay.
'knock' becomes 'rap' (synonyms).
'out' is an insertion indicator.
'the head of' indicates taking the first letters.
The initial letter of 'security' is 's'.
'rap' placed around 's' is 'RASP'.
'with a' is the link.
(Other definitions for rasp that I've seen before include "Smoother" , "unpleasant noise" , "Coarse file or grating sound" , "Grate; scrape" , "sort of berry?" .)