How can a FISO give a clearance? Is there exceptions?
The owner of the airfield can inpose any conditions he likes, it's his airfield and you use it on his terms.
Go read Rule 40 and Rule 41.
"Clearance" is not a defined term in the ANO or ROAR.
I imagine you are confusing "clearance" with "Air Traffic Control Clearance", which is a defined term
(b) ‘air traffic control clearance’ means an authorisation by an air traffic control unit for an aircraft to proceed under conditions specified by that unit;
You would have been correct had you suggested that a FISO could not issue an air traffic control clearance.