One called initially to step up to create glacial mass (3,3)
I believe the answer is:
ice cap
'glacial mass' is the definition.
'one called initially to step up' is the wordplay.
'one' becomes 'i' (Roman numeral).
'initially' indicates taking the first letters.
'to step' becomes 'pace' ('pace' can be a synonym of 'step'**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The initial letter of 'called' is 'c'.
'pace' in reverse letter order is 'ecap'.
'i'+'c'+'ecap'='ICE-CAP'
'to create' acts as a link.
(Other definitions for ice cap that I've seen before include "Permanently frozen mountain-top mass" , "Cold mass permanently covering the polar regions or mountain peaks" , "Permanent frozen polar cover" , "Permanently frozen covering of land" , "one providing cover for Pole" .)