First of skiers immediately departs on mountain (7)
I believe the answer is:
snowdon
'mountain' is the definition.
(Mount Snowdon in Wales)
'first of skiers immediately departs on' is the wordplay.
'first of skiers' becomes 's' (1st letter of 'skiers').
'immediately' becomes 'now' (I've seen this in other clues).
'departs' becomes 'd' (common abbreviation e.g. train timetables).
's'+'now'+'d'+'on'='SNOWDON'
(Other definitions for snowdon that I've seen before include "Highest peak in Wales" , "Highest mountain in Wales" , "Welsh mountain" , "that is looked up to in Wales" , "Highest Welsh peak" .)