Obscene sort of weld (4)
I believe the answer is:
lewd
'obscene' is the definition.
(I know that obscene can be written as lewd)
'sort of weld' is the wordplay.
'sort of' indicates anagramming the letters.
'weld' is an anagram of 'LEWD'.
(Other definitions for lewd that I've seen before include "Dirty" , "Bawdy" , "Indecent and vulgar (4)" , "Licentious or obscene" , "Salacious" .)