Song period mostly covered by book — inferior revised thesis (5,4,5)
I believe the answer is:
bless this house
'song period' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'mostly covered by book inferior revised thesis' is the wordplay.
'mostly' becomes 'hou' (I can't explain this - if you can you should give a lot more credence to this answer).
'covered by' indicates putting letters inside.
'book' becomes 'b' (abbreviation for book).
'inferior' becomes 'less' (lesser or less significant).
'revised' indicates an anagram.
'thesis' with letters rearranged gives '[thesis]'.
'b'+'less'+'[thesis]'='bless[thesis]'
'hou' going into 'blessthisse' is 'BLESS THIS HOUSE'.
Can you help me to learn more?