Mother's nag about missing lid on sage sauce (10)
I believe the answer is:
mayonnaise
'sage sauce' is the definition.
'mayonnaise' can be an answer for 'sauce' (mayonnaise is a kind of sauce). I'm unsure of the 'sage' bit.
'mother's nag about missing lid' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'nag' becomes 'annoy' (nag can mean to annoy or badger).
'about' says the letters should be written in reverse.
'missing lid' becomes 'ise' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'annoy' reversed gives 'yonna'.
'ma'+'yonna'+'ise'='MAYONNAISE'
'on' acts as a link.
(Other definitions for mayonnaise that I've seen before include "Creamy dressing" , "Egg/oil sauce" , "7 for salad" , "Salad dressing" , "which is saucy" .)