Freshwater reptile bringing one salmon up in ten (8)
I believe the answer is:
terrapin
'freshwater reptile' is the definition.
(I've seen this before)
'one salmon up in ten' is the wordplay.
'one' becomes 'i' (Roman numeral).
'salmon' becomes 'parr' (I've seen this before**).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' is an insertion indicator.
'i'+'parr'='iparr'
'iparr' in reverse letter order is 'rrapi'.
'rrapi' going inside 'ten' is 'TERRAPIN'.
'bringing' acts as a link.
(Other definitions for terrapin that I've seen before include "Chelonian reptile" , "Hardback" , "freshwater creature" , "A reptile; a printer (anag.)" , "Little turtle" .)