Swimmer brings back one young salmon in ten (8)
I believe the answer is:
terrapin
'swimmer' is the definition.
(I know that terrapin is a type of terrapin)
'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 in another clue).
'in' means one lot of letters goes inside another.
'i'+'parr'='iparr'
'iparr' in reverse letter order is 'rrapi'.
'rrapi' inserted inside 'ten' is 'TERRAPIN'.
'brings' is the link.
(Other definitions for terrapin that I've seen before include "Chelonian reptile" , "Small freshwater turtle" , "edible, freshwater reptile" , "Aquatic tortoise" , "freshwater creature" .)