Final answer:
The context clue to define 'Chary' is contrast, comparing Matt's excitement to his mother's cautiousness, indicating that 'Chary' means being careful or wary.
Step-by-step explanation:
The type of context clue that would help define the word Chary in the sentence 'Matt excitedly jumped into his dad’s new pick-up truck, while his mother was Chary about getting in' is contrast. The contrast is between Matt's excitement and his mother's hesitation. This suggests that 'Chary' means being cautious or wary.