Mounting trouble besetting Spain's old theatres (4)
I believe the answer is:
odea
'old theatres' is the definition.
(I've seen this before)
'mounting trouble besetting spain's' is the wordplay.
'mounting' shows that the letters should be reversed in order (going up in a down clue).
'trouble' becomes 'ado' (I've seen this before).
'besetting' is an insertion indicator.
'spain' becomes 'e' (abbreviation for Espana).
'ado' placed around 'e' is 'aedo'.
'aedo' in reverse letter order is 'ODEA'.
(Other definitions for odea that I've seen before include "halls giving entertainment" , "Old theatres" .)