Irving's first novel's last line is off-colour (3)
I believe the answer is:
ill
'off-colour' is the definition.
(off-colour can mean unwell)
'irving's first novel's last line' is the wordplay.
'irving's first' becomes 'i' (1st letter of 'irving').
'last' indicates one should take the final letters.
'line' becomes 'l' (used when specifying particular lines from a poem).
The last letter of 'novel' is 'l'.
'i'+'l'+'l'='ILL'
'is' is the link.
(Other definitions for ill that I've seen before include "Unwell, sick" , "Groggy" , "Green round the gills" , "Faulty" , "Sickly" .)