Trendy joint incorporates, oddly, this design (9)
I believe the answer is:
intention
'design' is the definition.
(thesaurus)
'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'.
'in'+'tention'='INTENTION'
(Other definitions for intention that I've seen before include "Aim, objective" , "Design" , "end" , "goal" , "Plan, purpose" .)