Extent of leak by head of security (5)
I believe the answer is:
sweep
'extent' is the definition.
(I know that sweep is a type of ambit)
'leak by head of security' is the wordplay.
'leak' becomes 'weep' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'head of' says to take the initial letters.
The first letter of 'security' is 's'.
'weep' put after 's' is 'SWEEP'.
'of' acts as a link.
(Other definitions for sweep that I've seen before include "Dust" , "Raffle" , "Victorian child labourer" , "Expanse" , "Brush; rush" .)