Silly old ratbag wrote at last to cancel (8)
I believe the answer is:
abrogate
'cancel' is the definition.
(thesaurus)
'silly old ratbag wrote at last' is the wordplay.
'silly' indicates anagramming the letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'at last' says to take the final letters.
The final letter of 'wrote' is 'e'.
'o'+'ratbag'='oratbag'
'oratbag' is an anagram of 'abrogat'.
'abrogat'+'e'='ABROGATE'
'to' acts as a link.
(Other definitions for abrogate that I've seen before include "Cancel or repeal, say a law" , "End" , "Get a boar to annul" , "Annul or revoke" , "Abolish formally" .)