Everyone with great person is full of energy and loyal (9)

'loyal' is the definition.
('allegiant' can be similar in meaning to 'loyal')

'everyone with great person is full of energy' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'with' means one lot of letters go next to another.
'great person' becomes 'giant' (giant is a kind of person. I am not sure about the 'great' bit.**).
'is full of' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mci).
'all'+'giant'='allgiant'
'allgiant' going around 'e' is 'ALLEGIANT'.

'and' acts as a link.

Can you help me to learn more?