Secure gear climbing first in Dolomites (4)
I believe the answer is:
gird
'secure' is the definition.
(to gird is to secure something with a belt)
'gear climbing first in dolomites' is the wordplay.
'gear' becomes 'rig' (rig is a kind of gear**).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'first in' suggests taking the first letters.
The first letter of 'dolomites' is 'd'.
'rig' backwards is 'gir'.
'gir'+'d'='GIRD'
(Other definitions for gird that I've seen before include "ring" , "Steel" , "Put a belt on" , "Surround" , "Encircle - fasten with belt" .)