Trendy joint incorporates, oddly, this design (9)
I believe the answer is:
Here is my best explanation:
'design' is the definition.
'trendy joint incorporates oddly this' is the subsidiary indication.
'trendy' becomes 'in'.
'joint' becomes 'tenon' (I've seen this in other clues**).
'incorporates' means one lot of letters goes inside another.
'oddly' means one should take alternating letters.
The alternating letters of 'this' are 'ti'.
'tenon' placed around 'ti' is 'tention'.
(Other definitions for intention that I've seen before include "Purpose, aim", "Design", "end", "goal", "Aim, purpose", "End", "Objective", "Aim, objective", "Plan, purpose" and "Aim, plan".)