Being very young, is upset and cross (3)
I believe the answer is:
six
'being very young is upset and cross' is the definition.
I can't judge whether this definition defines the answer.
'is upset and cross' is the wordplay.
'upset' says the letters should be written backwards.
'and' says to put letters next to each other.
'cross' becomes 'x' (X is cross-shaped).
'is' in reverse letter order is 'si'.
'si'+'x'='SIX'
Can you help me to learn more?
(Other definitions for six that I've seen before include "Number of bowls in a cricket over" , "Number of US moon landings" , "Airborne boundary hit" , "Maximum cricket hit" , "VI" .)