Mostly carry young child (3)
I believe the answer is:
tot
'young child' is the definition.
(I've seen this before)
'mostly carry' is the wordplay.
'mostly' means to remove the last letter.
'carry' becomes 'tote' (toting is a kind of carrying).
'tote' with its last letter taken away is 'TOT'.
(Other definitions for tot that I've seen before include "Add; young child" , "Child's" , "Calculate" , "Tiny child" , "Add up for baby" .)