Shop for writers where trains are — supported by Queen (9)
I believe the answer is:
stationer
'shop for writers' is the definition.
(a stationer sells pens)
'where trains are supported by queen' is the wordplay.
'where trains are' becomes 'station' (trains pull into a station).
'supported by' says to put letters next to each other (in a down clue, some letters hold up others).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'station'+'er'='STATIONER'
(Other definitions for stationer that I've seen before include "bookseller" , "has what writer needs" , "Poster" , "One assigning position" , "Shopkeeper" .)