Monarch in Balmoral perhaps, and mountainous Welsh location (10)
I believe the answer is:
caerphilly
'welsh location' is the definition.
'monarch in balmoral perhaps and mountainous' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen).
'in' indicates putting letters inside.
'balmoral perhaps' becomes 'cap' (balmoral is a kind of cap).
'and' means one lot of letters go next to another.
'mountainous' becomes 'hilly' (synonyms).
'er' going inside 'cap' is 'caerp'.
'caerp'+'hilly'='CAERPHILLY'
(Other definitions for caerphilly that I've seen before include "Welsh town" , "Town north of Cardiff" , "Mild white cheese" .)