Small dog or cat, perhaps - it has energy (6)
I believe the answer is:
petite
'small' is the definition.
(similar in meaning)
'dog or cat perhaps it has energy' is the wordplay.
'dog or cat' becomes 'pet' (I've seen this in another clue).
'perhaps' indicates anagramming the letters.
'has' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mce).
'it' after 'e' is 'eit'.
'eit' is an anagram of 'ite'.
'pet'+'ite'='PETITE'
(Other definitions for petite that I've seen before include "Elfin" , "Short" , "Attractively small" , "Little French" , "Small and attractive" .)