One shifting around ice as fish start to rot (6)
I believe the answer is:
skater
'one shifting around ice' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'fish start to rot' is the wordplay.
'fish' becomes 'skate' (skate is a kind of fish).
'start to' suggests taking the first letters.
The initial letter of 'rot' is 'r'.
'skate'+'r'='SKATER'
'as' is the link.
Can you help me to learn more?
(Other definitions for skater that I've seen before include "Rink performer" , "Could one roll up" , "Dean, perhaps" , "One sliding over ice" , "Streak (anag)" .)