Small present put on island by American in national park (9)
I believe the answer is:
snowdonia
'national park' is the definition.
(Welsh national park)
'small present put on island by american' is the wordplay.
'small present' becomes 'snowdon' (I am not sure about this - if you are sure you should believe this answer much more).
'put on' means one lot of letters go next to another.
'island' becomes 'i'.
'by' says to put letters next to each other.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'snowdon'+'i'+'a'='SNOWDONIA'
'in' acts as a link.
(Other definitions for snowdonia that I've seen before include "peaks" , "National park in Wales" , "mountains" , "Win a snood in the Welsh county" , "somewhere in 10" .)