Half-mild one; moreover half-cheerful (6)
I believe the answer is:
genial
'cheerful' is the definition.
(I've seen this before)
'half-mild one moreover half' is the wordplay.
'half' suggests halving the letters.
'mild' becomes 'gentle' (similar in meaning).
'one moreover half' becomes 'ial' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gentle' halved is 'gen'.
'gen'+'ial'='GENIAL'
Can you help me to learn more?
(Other definitions for genial that I've seen before include "'Affable, warm and friendly (6)'" , "Friendly and agreeable" , "Angel I found being affable" , "Affable in Ealing" , "Cheerful and well-disposed" .)