Timid people nursing essentially stealthy ill-will (6)
I believe the answer is:
malice
'ill-will' is the definition.
(I've seen this before)
'timid people nursing essentially stealthy' is the wordplay.
'timid people' becomes 'mice' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'nursing' means one lot of letters goes inside another.
'essentially' indicates the central letters.
The middle letters of 'stealthy' are 'al'.
'mice' placed around 'al' is 'MALICE'.
Can you help me to learn more?
(Other definitions for malice that I've seen before include "Spiteful, animosity, sometimes a forethought" , "Desire to harm others, may be aforethought" , "Spite - desire to hurt" , "Hate" , "Ill will, a desire to harm others" .)