Senseless boss entertains swine no end (6)
I believe the answer is:
stupid
'senseless' is the definition.
(similar in meaning)
'boss entertains swine no end' is the wordplay.
'boss' becomes 'stud' (synonyms).
'entertains' indicates putting letters inside.
'swine' becomes 'pig' (pig is a kind of swine).
'no end' means to remove the last letter.
'pig' with its last letter taken off is 'pi'.
'stud' enclosing 'pi' is 'STUPID'.
(Other definitions for stupid that I've seen before include "moronic" , "Not intelligent, thick" , "Not very bright" , "Very silly" , "Lacking in common sense" .)