Queen (in hat) reaching mountainous Welsh location (10)
I believe the answer is:
caerphilly
'welsh location' is the definition.
'queen in hat reaching mountainous' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'in' indicates putting letters inside.
'hat' becomes 'cap' (I've seen this before**).
'reaching' means one lot of letters go next to another.
'mountainous' becomes 'hilly' ('hilly' can be a synonym of 'mountainous').
'er' placed inside 'cap' is 'caerp'.
'caerp'+'hilly'='CAERPHILLY'
(Other definitions for caerphilly that I've seen before include "Mild white cheese" , "Welsh town" , "Town north of Cardiff" .)