Frozen French lake -- one eroded in hollow ground ... (9)

'frozen' is the definition.
(I've seen this in another clue)

'french lake one eroded in hollow ground' is the wordplay.
'french lake' becomes 'lac' ('lake' in French).
'one' becomes 'i' (Roman numeral).
'eroded' becomes 'ate' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'hollow' means to remove the middle letters (the word is hollowed out).
'ground' with its middle removed is 'gd'.
'lac'+'i'+'ate'='laciate'
'laciate' put inside 'gd' is 'GLACIATED'.