1,000 passing through change for Slough (5)
I believe the answer is:
swamp
'for slough' is the definition.
(I've seen this before)
'1000 passing through change' is the wordplay.
's000' becomes 'M' (roman numeral).
'passing through' is an insertion indicator.
'change' becomes 'swap' (swapping is a kind of changing).
'm' inserted into 'swap' is 'SWAMP'.
(Other definitions for swamp that I've seen before include "In flood" , "Waterlogged ground" , "Slough" , "Overwhelm; wetland" , "Bog, marsh" .)