Child, say, in part of pool with lower energy, not disheartened (9)
I believe the answer is:
dependent
'child say in part of pool' is the definition.
I can't tell whether this defines the answer.
'lower energy not disheartened' is the wordplay.
'lower' becomes 'depend' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'energy' becomes 'e' (physics symbol as in E=mce).
'disheartened' suggests removing the centre (the middle or heart is removed).
'not' with its centre taken out is 'nt'.
'depend'+'e'+'nt'='DEPENDENT'
'with' is the link.
Can you help me to learn more?
(Other definitions for dependent that I've seen before include "Subject" , "Relying/contingent (on)" , "Reliant" .)