Swimmer in Moselle on mobile! (5,4)
I believe the answer is:
lemon sole
'swimmer' is the definition.
(lemon sole is a kind of fish)
'moselle on mobile' is the wordplay.
'mobile' indicates an anagram (mobile can mean moving).
'moselle'+'on'='moselleon'
'moselleon' with letters rearranged gives 'LEMON-SOLE'.
'in' acts as a link.
(Other definitions for lemon sole that I've seen before include "Flatfish, valued as food" , "Common flatfish" , "Tasty plaice of citrus origin?" , "Another tasty flatfish like lone citrus fruit" , "Edible flatfish" .)