Mother fast to catch rodent -- nasty! (10)
I believe the answer is:
malevolent
'nasty' is the definition.
(thesaurus)
'mother fast to catch rodent' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'fast' becomes 'lent' (I've seen this before).
'to catch' 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 "Spiteful" , "bitter" , "Wishing evil to others" , "Ill-disposed towards others" , "very bad" .)