Moderate rage first – get it? (3,3,5)
I believe the answer is:
see the light
'get it?' is the definition.
(I've seen this before)
'moderate rage first' is the wordplay.
'moderate rage' becomes 'seethe'.
'first' becomes 'light' (I can't explain this - if you can you should give a lot more credence to this answer).
'seethe'+'light'='SEE THE LIGHT'
Can you help me to learn more?
(Other definitions for see the light that I've seen before include "Realise a fact or mistake - be religiously converted" , "Gain sudden understanding" , "Realise one's mistake" , "Come to understand" , "Be born" .)