Crafty nurses understanding only a little (8)
I believe the answer is:
Here is my best explanation:
'only a little' is the definition.
(I've seen this before)
'crafty nurses understanding' is the subsidiary indication.
'crafty' becomes 'sly' ('sly' can be a synonym of 'crafty').
'nurses' is an insertion indicator.
'understanding' becomes 'light' (light is a kind of understanding).
'sly' going around 'light' is 'slightly'.
(Other definitions for slightly that I've seen before include "Mildly", "To a small degree", "To a small extent", "little bit", "Quite!", "Not very much", "Only a little", "Somewhat", "A little" and "a tiny bit".)