Climbing mountains, call to be extravagant (6,3)
I believe the answer is:
splash out
'extravagant' is the definition.
(I've seen this before)
'climbing mountains call' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'mountains' becomes 'alps' (European mountain range).
'call' becomes 'shout' (synonyms).
'alps' written backwards gives 'spla'.
'spla'+'shout'='SPLASH OUT'
'to be' is the link.
(Other definitions for splash out that I've seen before include "Spend a lot of money" , "extravagant" , "Blow brass" , "Spend money freely" .)