Pen in support of form and decency (7)
I believe the answer is:
modesty
'decency' is the definition.
(I know that modesty is a type of decency)
'pen in support of form' is the wordplay.
'pen' becomes 'sty' (both can mean an animal enclosure).
'in support of' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'form' becomes 'mode' (I've seen this in another clue).
'sty' put after 'mode' is 'MODESTY'.
'and' is the link.
(Other definitions for modesty that I've seen before include "lack of vanity" , "Unassuming quality" , "Way to get disorderly house under" , "Unpretentious style" , "Quality of being unpretentious" .)