Some loathsome swearing (4)
I believe the answer is:
oath
'swearing' is the definition.
(synonyms)
'some loathsome' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'OATH' is hidden within 'loathsome'.
(Other definitions for oath that I've seen before include "Strong language" , "A curse or a vow" , "Solemn pledge" , "blasphemy" , "A solemn undertaking to tell the truth" .)