Architects presented with line drawing originally blue (6)
I believe the answer is:
ribald
'blue' is the definition.
('ribald' can be a synonym of 'blue')
'architects presented with line drawing originally' is the wordplay.
'architects' becomes 'riba' (Royal Institute of British Architects).
'presented with' means one lot of letters go next to another.
'originally' says to take the initial letters.
The first letters of 'line drawing' is 'ld'.
'riba'+'ld'='RIBALD'
(Other definitions for ribald that I've seen before include "Mean" , "Humorous in vulgar or disrespectful way" , "Coarsely humorous; scurrilous" , "Funny, but not nice" , "Scurrilous, bawdy" .)