Lacking a centre of gravity? (12)
I believe the answer is:
'lacking a' is the definition.
I can't tell whether this definition defines the answer.
'centre of gravity?' is the wordplay.
I cannot really understand how this works, but
'centre' could be 'heart' (synonyms) and 'heart' is located in the answer.
'gravity?' could be 'g' and 'g' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for lighthearted that I've seen is " Cheerful".)