Some loathsome swearword (4)
I believe the answer is:
oath
'swearword' is the definition.
(oath can mean a swearword)
'some loathsome' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'OATH' is hidden within 'loathsome'.
(Other definitions for oath that I've seen before include "'Profane, or solemn expression (4)'" , "It could be ruddy when minced" , "Swearword or solemn promise" , "word intended?" , "Imprecation" .)