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 wordplay.
'trendy' becomes 'in'.
'joint' becomes 'tenon' (I've seen this before**).
'incorporates' means one lot of letters goes inside another.
'oddly' indicates alternate 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".)