Huge stone finally going in the mail, remarkably (8)
I believe the answer is:
megalith
'huge stone' is the definition.
(I've seen this before)
'finally going in the mail remarkably' is the wordplay.
'finally' suggests the final letters.
'in' is an insertion indicator.
'remarkably' indicates an anagram.
The last letter of 'going' is 'g'.
'the'+'mail'='themail'
'themail' with letters rearranged gives 'mealith'.
'g' placed inside 'mealith' is 'MEGALITH'.
(Other definitions for megalith that I've seen before include "Upright (prehistoric) stone" , "Huge ancient monument stone" , "Large stone in a prehistoric monument" , "Huge stone used in prehistoric monuments" .)