Way off, just terribly constructed? (5-5)
I believe the answer is:
jerry-built
'way off just' is the definition.
I can't tell whether this definition defines the answer.
'terribly constructed?' is the wordplay.
'terribly' becomes 'jerry' (I can't justify this - if you can you should give a lot more credence to this answer).
'constructed?' becomes 'built' ('build' can be a synonym of 'construct').
'jerry'+'built'='JERRY-BUILT'
(Other definitions for jerry-built that I've seen before include "(Of housing) made from cheap materials" , "Bad or hastily constructed" , "(Of house construction) flimsy" , "(Of houses) constructed to inferior standards" .)