Boatman, a rear admiral, climbing a mountain (6)
I believe the answer is:
ararat
'a mountain' is the definition.
(Turkish mountain)
'boatman a rear admiral climbing' is the wordplay.
'boatman' becomes 'tar' (tar is an informal term for a sailor).
'rear admiral' becomes 'RA' (naval abbreviation).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'tar'+'a'+'ra'='tarara'
'tarara' back-to-front is 'ARARAT'.
(Other definitions for ararat that I've seen before include "Turkey's highest mountain" , "OT mount" , "Mount where Noah's Ark came to rest" , "Traditional landing place of Noah's ark" , "Mount on which Noah's ark grounded" .)