Seemliness in the fashion pen (7)
I believe the answer is:
modesty
'seemliness' is the definition.
(thesaurus)
'fashion pen' is the wordplay.
'fashion' becomes 'mode' (synonyms).
'pen' becomes 'sty' (both can mean an animal enclosure).
'mode'+'sty'='MODESTY'
'in the' acts as a link.
(Other definitions for modesty that I've seen before include "Unassumingness" , "Reserve" , "Unpretentious style" , "Diffidence" , "Decorum, shyness" .)