Reptile brings back one young salmon in ten (8)
I believe the answer is:
terrapin
'reptile' is the definition.
(terrapin is a kind of reptile)
'back one young salmon in ten' is the wordplay.
'back' says the letters should be written backwards.
'one' becomes 'i' (Roman numeral).
'young salmon' becomes 'parr' (I've seen this before).
'in' means one lot of letters goes inside another.
'i'+'parr'='iparr'
'iparr' backwards is 'rrapi'.
'rrapi' put into 'ten' is 'TERRAPIN'.
'brings' is the link.
(Other definitions for terrapin that I've seen before include "edible, freshwater reptile" , "Swimmer" , "Small freshwater turtle" , "Water tortoise" , "Small species of turtle" .)