A saying about limiting consumption, fundamentally (2,6)
I believe the answer is:
at bottom
'fundamentally' is the definition.
(I've seen this in another clue)
'a saying about limiting consumption' is the wordplay.
'saying' becomes 'motto' (motto is a kind of saying).
'about' says the letters should be written in reverse.
'limiting' means one lot of letters goes inside another.
'consumption' becomes 'tb' (consumption is a name for TB or tuberculosis).
'motto' in reverse letter order is 'ottom'.
'a'+'ottom'='aottom'
'aottom' enclosing 'tb' is 'AT BOTTOM'.
(Other definitions for at bottom that I've seen before include "Basically" , "Like the Titanic" , "Location of 27 28" .)