Mum keeps a small car: it sometimes runs (7)
I believe the answer is:
mascara
'it sometimes runs' is the definition.
The definition suggests a singular noun which matches the answer.
'mum keeps a small car' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'+'car'='ascar'
'ma' enclosing 'ascar' is 'MASCARA'.
Can you help me to learn more?
(Other definitions for mascara that I've seen before include "Type of make-up" , "Eyelash make-up" , "Cosmetic for darkening the eyelashes" , "that accentuates lashes" , "Cosmetic for the eyes" .)