'Honestly, no energy' is start of matrimonial platitude (6)
I believe the answer is:
truism
'matrimonial platitude' is the definition.
'truism' can be an answer for 'platitude' (thesaurus). I'm not certain of the 'matrimonial' bit.
'honestly no energy is start' is the wordplay.
'honestly' becomes 'true' (can be an adverb as in 'speak true', 'ring true').
'no' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mcs).
'is start' becomes 'ism' (I can't explain this - if you can you should give a lot more credence to this answer).
'true' with 'e' removed is 'tru'.
'tru'+'ism'='TRUISM'
'of' is the link.
Can you help me to learn more?
(Other definitions for truism that I've seen before include "An obvious statement that says noting new or interesting" , "Statement or fact of obvious veracity" , "Something obvious" , "self-evident statement" , "Obviously correct and verified statement" .)