Fish advanced into Scottish waters (5)
I believe the answer is:
loach
'fish' is the definition.
(loach is a kind of fish)
'advanced into scottish waters' is the wordplay.
I cannot quite see how this works, but
'advanced' could be 'a' (abbreviation) and 'a' is present in the answer.
The remaining letters 'loch' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for loach that I've seen before include "Fish with barbels around mouth" , "River fish - film director" , "Freshwater fish; rhymes with freshwater fish" , "Fish - British film director" , "Film-maker" .)