Long to have costume in certain design (9)
I believe the answer is:
pinstripe
'long to have costume in certain design' is the definition.
I can't tell whether this defines the answer.
'long to have costume in certain design' is the wordplay.
'long' becomes 'pine' (to pine for someone is to long for them).
'to have' means one lot of letters goes inside another.
'costume in certain design' becomes 'strip' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pine' enclosing 'strip' is 'PINSTRIPE'.
Can you help me to learn more?
(Other definitions for pinstripe that I've seen before include "maybe a suit" , "Cloth for making suits" , "material" , "business suit" , "Fabric - insert pip (anag)" .)