Force rips off part of dress (9)
I believe the answer is:
'force' is the definition.
(I've seen this before)
'rips off part of dress' is the wordplay.
'rips off' becomes 'cons' (conning is a kind of ripping off).
'part of dress' becomes 'train' (I can't justify this - if you can you should believe this answer much more).
Can you help me to learn more?
(Other definitions for constrain that I've seen before include "Restrict scope of (an activity)" , "Force" , "Confine, limit" , "Compel; constrict" , "Compel - limit" .)