Large permanently frozen area, for example at the poles (6)
(Other definitions for ice cap that I've seen before include "Frozen polar region" , "one providing cover for Pole" , "Permanent covering of frozen water" , "Permanently frozen covering of land" , "Polar feature" .)
(Other definitions for icecap that I've seen before include "Frozen top of mountain etc" , "Cold mass permanently covering the polar regions or mountain peaks" , "Permanently frozen polar mass" , "Permanent covering of frozen water" , "Permanent coating of white on a mountain-top" .)