Lows include the core of a hundred states of mind (5)
I believe the answer is:
moods
'states of mind' is the definition.
(I've seen this in another clue)
'lows include the core of a hundred' is the wordplay.
'lows' becomes 'moos' ('moo' can be a synonym of 'low').
'include' means one lot of letters goes inside another.
'the core of' means to look at the middle letters.
The middle of 'hundred' is 'd'.
'moos' enclosing 'd' is 'MOODS'.
(Other definitions for moods that I've seen before include "Temporary states of mind or feelings" , "Humours, feelings" , "Dooms to fits of depression" .)