Mostly dice with young of the species (3)
I believe the answer is:
cub
'young of the species' is the definition.
'cub' can be an answer for 'young' (cub is a kind of young). I'm not sure about the 'of the species' bit.
'mostly dice' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'dice' becomes 'cube' (synonyms).
'cube' with its last letter taken away is 'CUB'.
'with' is the link.
Can you help me to learn more?
(Other definitions for cub that I've seen before include "Young one" , "Young of certain animals" , "Whelp" , "Young animal or scout aspirant" , "'Baby animal, fox say (3)'" .)