Mountain on which an accountant captured animal (6)
I believe the answer is:
alpaca
'animal' is the definition.
(South American animal)
'mountain on which an accountant captured' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'on which' says to put letters next to each other.
'an' becomes 'a'.
'accountant' becomes 'ac'.
'captured' means one lot of letters goes inside another.
'alp'+'a'='alpa'
'alpa' going around 'ac' is 'ALPACA'.
(Other definitions for alpaca that I've seen before include "South American mammal" , "Cloth made from wool of S American llama" , "South American camelid" , "Llama relative, valued for it's wool" , "Break down (food)" .)