Use up colour infusing gelato evenly (3,4)
I believe the answer is:
eat into
'use up' is the definition.
(synonyms)
'colour infusing gelato evenly' is the wordplay.
'colour' becomes 'tint' (tint is a kind of colour).
'infusing' indicates putting letters inside (to infuse is to fill or enter).
'evenly' indicates alternate letters (the letters in even-numbered positions).
The alternating letters of 'gelato' are 'eao'.
'tint' going within 'eao' is 'EAT INTO'.
(Other definitions for eat into that I've seen before include "start wearing out" , "Etch" , "run down" , "Consume (resources)" , "Corrode (something)" .)