Weak understanding (5)
I believe the answer is:
light
This clue is a double definition.
'weak' is the first definition.
(synonyms)
'understanding' is the second definition.
(light is a kind of understanding)
(Other definitions for light that I've seen before include "guide in the dark" , "Match - agile - easily digested" , "Mild" , "Frivolous -- insubstantial" , "Not too demanding" .)