Huge mass seen twice in turbulent Seine (7)
I believe the answer is:
immense
'huge' is the definition.
(I know that huge can be written as immense)
'mass seen twice in turbulent seine' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'seen twice' shows that letters should be duplicated.
'in' means one lot of letters goes inside another.
'turbulent' is an anagram indicator.
'm' doubled is 'mm'.
'seine' with letters rearranged gives 'iense'.
'mm' placed inside 'iense' is 'IMMENSE'.
(Other definitions for immense that I've seen before include "Jumbo" , "Vast or enormous" , "Unctuously ingratiating" , "Unusually great in size" , "Massive" .)