One charged entry to Adventure Island (4)
I believe the answer is:
iona
'island' is the definition.
(Scottish island)
'one charged entry to adventure' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
The remaining letters 'ona' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for iona that I've seen before include "Isle of the Inner Hebrides, made famous by St Columba" , "Site of St Columba's monastery" , "place in the Hebrides" , "holy site" , "Isolated part of UK" .)