Spied new telegram that's loathsome (10)
I believe the answer is:
despicable
'that's loathsome' is the definition.
(thesaurus)
'spied new telegram' is the wordplay.
'new' indicates anagramming the letters.
'telegram' becomes 'cable' (cable is a kind of telegram).
'spied' is an anagram of 'despi'.
'despi'+'cable'='DESPICABLE'
(Other definitions for despicable that I've seen before include "rotten" , "Vile" , "bum" , "Mean" .)