Rock climbing in secret in Argentina (7)
I believe the answer is:
granite
'rock' is the definition.
(type of rock)
'climbing in secret in argentina' is the wordplay.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'in' indicates the answer is hidden within the clue.
'etinarg' is hidden in the letters of 'secret in argentina'.
'etinarg' in reverse letter order is 'GRANITE'.
(Other definitions for granite that I've seen before include "Get rain (anag) - rock" , "Hard igneous rock" , "Coarse-grained igneous rock" , "Igneous rock, generally composed of feldspar and mica and quartz" , "Hard granular rock" .)