Fish by river: one to glide across frozen surface (6)
I believe the answer is:
skater
'one to glide across frozen surface' is the definition.
I can't judge whether this definition defines the answer.
'fish by river' is the wordplay.
'fish' becomes 'skate' (skate is a kind of fish).
'by river' becomes 'r' (abbreviation for river. I am not sure about the 'by' bit.).
'skate'+'r'='SKATER'
Can you help me to learn more?
(Other definitions for skater that I've seen before include "One sliding over ice" , "Streak (anag)" , "One moving over ice" , "Ice sportsman" , "Winter sports participant" .)