Lower level in Scottish river (7)
I believe the answer is:
deflate
'lower' is the definition.
(thesaurus)
'level in scottish river' is the wordplay.
'level' becomes 'flat' ('flat' can be a synonym of 'level'**).
'in' means one lot of letters goes inside another.
'scottish river' becomes 'dee' (I've seen this before).
'flat' put within 'dee' is 'DEFLATE'.
(Other definitions for deflate that I've seen before include "Puncture, collapse like balloon" , "Release gas from" , "Cause to shrink or collapse" , "Let the wind out of, braggart or balloon" , "Release air from" .)