Swears it's a fine company on top of security (7)
I believe the answer is:
affirms
'swears' is the definition.
('affirm' can be a synonym of 'swear')
'a fine company on top of security' is the wordplay.
'a fine' becomes 'af' (I've seen this in other clues).
'company' becomes 'firm' (I've seen this before).
'on top of' indicates taking the first letters (I've seen 'on top' mean this).
The first letter of 'security' is 's'.
'af'+'firm'+'s'='AFFIRMS'
'it's' is the link.
(Other definitions for affirms that I've seen before include "States - swears" , "Declares" .)