City's river enclosed in sheet of ice (8)
I believe the answer is:
florence
'city's' is the definition.
(Florence is an example)
'river enclosed in sheet of ice' is the wordplay.
'river' becomes 'r' (abbreviation).
'enclosed' becomes 'enc' (abbreviation for 'enclosed' as in letters).
'in' means one lot of letters goes inside another.
'sheet of ice' becomes 'floe' (I've seen this in another clue).
'r'+'enc'='renc'
'renc' placed within 'floe' is 'FLORENCE'.
(Other definitions for florence that I've seen before include "Italian city, noted art centre" , "Angel of the reclining (anag) -- nurse" , "The familiar Miss Nightingale, famous nurse" , "___ Nightingale, the Lady with the Lamp, d. 1910" , "place in Italy" .)