Blair cracking tedious, off-colour jokes, say (8)
I believe the answer is:
ribaldry
'off-colour jokes say' is the definition.
(ribaldry is rude humour)
'blair cracking tedious' is the wordplay.
'cracking' indicates an anagram.
'tedious' becomes 'dry' (I've seen this in other clues).
'blair' with letters rearranged gives 'ribal'.
'ribal'+'dry'='RIBALDRY'
(Other definitions for ribaldry that I've seen before include "Rabelaisian behaviour" , "Vulgar, indecent talk" , "Obscene language" , "Coarse and disrespectful humour" , "unseemly mirth" .)