Fish by Scottish island, one with many tongues? (8)
I believe the answer is:
linguist
'one with many tongues?' is the definition.
(person working with languages or 'tongues')
'fish by scottish island' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'by' is a charade indicator (letters next to each other).
'scottish island' becomes 'uist' (North or South Uist in the Outer Hebrides).
'ling'+'uist'='LINGUIST'
(Other definitions for linguist that I've seen before include "One proficient in languages" , "Person proficient in languages" , "Polyglot" , "Language expert" , "He speaks in tongues" .)