Mother's endless worry? Answer is cosmetic (7)
I believe the answer is:
mascara
'is cosmetic' is the definition.
(I've seen this before)
'mother's endless worry? answer' is the wordplay.
'mother's' becomes 'ma's' (ma can mean mother).
'endless' means to remove the last letter.
'worry?' becomes 'care' (eg without a care means without a worry).
'answer' becomes 'a' (abbreviation eg in Q and A).
'care' with its last letter taken off is 'car'.
'mas'+'car'+'a'='MASCARA'
(Other definitions for mascara that I've seen before include "Eyelash cosmetic" , "Type of make-up" , "facial cosmetic" , "Cosmetic for the eyes" , "Makeup used to darken and thicken the eye lashes" .)