Off-colour and about to enter shaft (4)
I believe the answer is:
racy
'off-colour' is the definition.
(both can mean risque or obscene)
'about to enter shaft' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to enter' is an insertion indicator.
'shaft' becomes 'ray' (synonyms).
'c' going into 'ray' is 'RACY'.
'and' acts as a link.
(Other definitions for racy that I've seen before include "Lively" , "Risqu" , "rather blue" , "Suggestive; spirited" , "Spicy" .)