Huge rear — about to slip into underwear, the biggest of its kind (7)
I believe the answer is:
everest
'the biggest of its kind' is the definition.
(Mount Everest is the world's highest mountain)
'huge rear about to slip into underwear' is the wordplay.
'rear' suggests the final letters (the back of the word).
'about' becomes 're' (regarding).
'to slip into' indicates putting letters inside.
'underwear' becomes 'vest'.
The final letter of 'huge' is 'e'.
're' going within 'vest' is 'verest'.
'e'+'verest'='EVEREST'
(Other definitions for everest that I've seen before include "The world's highest mountain" , "Tee revs up the mountain" , "Himalayan mount" , "A high spot" , "A mountain to climb" .)