Plan six hundred Scottish river bridges (6)
I believe the answer is:
device
'plan' is the definition.
(thesaurus)
'six hundred scottish river bridges' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'hundred' becomes 'c' (Roman numeral).
'scottish river' becomes 'dee' (I've seen this in another clue).
'bridges' means one lot of letters goes inside another (some letters go around or bridge others).
'vi'+'c'='vic'
'vic' going inside 'dee' is 'DEVICE'.
(Other definitions for device that I've seen before include "Contrivance; motto" , "Plan, scheme, trick" , "Plan - emblem" , "Appliance; scheme" , "Trick; heraldic design" .)