Small dog or cat, perhaps - it has energy (6)
'small' is the definition.
(similar in meaning)
'cat perhaps it has energy' is the wordplay.
'cat perhaps' becomes 'pet' (cat is a type of pet).
'has' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'dog or' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for petite that I've seen before include "Attractively small", "Wee", "Elfin", "Small, slim (girl)", "Little French".)