Start of Christmas with trading opportunity on the up for light units (8)

'light units' is the definition.
(candela is a kind of light unit)

'start of christmas with trading opportunity on the up' is the wordplay.
'start of' says to take the initial letters.
'with' becomes 'and'.
'trading opportunity' becomes 'sale' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on the up' says the letters should be written backwards (in a down clue, letters go up).
The initial letter of 'christmas' is 'c'.
'sale' back-to-front is 'elas'.
'c'+'and'+'elas'='CANDELAS'

'for' acts as a link.

Can you help me to learn more?