I'm to see rocks after heading for Yellowstone National Park (8)
I believe the answer is:
yosemite
'national park' is the definition.
(I've seen this before)
'i'm to see rocks after heading for yellowstone' is the wordplay.
'rocks' is an anagram indicator.
'after' is a charade indicator (letters next to each other).
'heading for' suggests taking the first letters.
The first letter of 'yellowstone' is 'y'.
'im'+'to'+'see'='imtosee'
'imtosee' anagrammed gives 'osemite'.
'osemite' put after 'y' is 'YOSEMITE'.
(Other definitions for yosemite that I've seen before include "Californian national park" , "US National Park in California" , "California falls, valley, park" , "Natl. Park in California" , "Area of beauty" .)