Clifton, 28, won one cap for Montserrat in 2015 and had a two-year spell at York Road between 2013 and 2015.
French defender Clerima, 27, joined Alan Devonshire's side last summer and also played under him at Braintree.
Midfielder McCormack, 18, was part of Reading's academy and was offered a new Maidenhead deal in May.
Find all the latest football transfers on our dedicated page.
National League side Maidenhead United have re-signed midfielder Adrian Clifton, while Remy Clerima and Sean McCormack have agreed new contracts.