Page in contract on a security system (7)

I believe the answer is:

captcha

'security system' is the definition.
(online security measure)

'page in contract on a' is the wordplay.
'page' becomes 'p'.
'in' is an insertion indicator.
'contract' becomes 'catch' (to catch an illness is to contract it).
'on' says to put letters next to each other.
'p' placed within 'catch' is 'captch'.
'captch'+'a'='CAPTCHA'

(Another definition for captcha that I've seen is " E-security feature".)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.