Spiteful mother fast to trap rodent (10)
I believe the answer is:
malevolent
'spiteful' is the definition.
(synonyms)
'mother fast to trap rodent' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'to trap' indicates putting letters inside.
'rodent' becomes 'vole' (vole is a kind of rodent).
'lent' enclosing 'vole' is 'levolent'.
'ma'+'levolent'='MALEVOLENT'
(Other definitions for malevolent that I've seen before include "Evil-minded" , "bitter" , "Having evil intentions" , "very bad" , "Wishing evil to others" .)