Very slow girl eats up single calorie (7)
I believe the answer is:
glacial
'very slow' is the definition.
(as in a glacial place)
'girl eats up single calorie' is the wordplay.
'girl' becomes 'gal' (synonyms).
'eats' means one lot of letters goes inside another.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'single' becomes 'I' (one).
'calorie' becomes 'cal'.
'i'+'cal'='ical'
'ical' in reverse letter order is 'laci'.
'gal' placed around 'laci' is 'GLACIAL'.
(Other definitions for glacial that I've seen before include "Freezing - unfriendly" , "Extremely cold" , "Icy; distant" , "Cold and icy" , "Frigid" .)