Send back papers before getting married, stupid! (3)
I believe the answer is:
dim
'stupid' is the definition.
(similar in meaning)
'send back papers before getting married' is the wordplay.
'send back' says the letters should be written in reverse.
'papers' becomes 'ID' (identification papers).
'before getting' says to put letters next to each other.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'id' backwards is 'di'.
'di'+'m'='DIM'
(Other definitions for dim that I've seen before include "Darken" , "Unpromising" , "Obscure, not bright" , "Not bright, poorly lit" , "Not bright, shadowy" .)