River creature from the south with thin bony scales (7)
I believe the answer is:
laminar
'river creature from the south with thin bony scales' is the definition.
I can't tell whether this defines the answer.
'creature from the south with thin bony scales' is the wordplay.
'creature' becomes 'animal' (synonyms).
'from the south' shows that the letters should be reversed in order (in a down clue, letters go up).
'with thin bony scales' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'animal' written backwards gives 'lamina'.
'lamina'+'r'='LAMINAR'
(Other definitions for laminar that I've seen before include "Like some thin plates" , "In sheet form" , "sheet upon sheet" , "Running in flat layers" , "Like non-turbulent flow" .)