Where ministers change top lines (6)
I believe the answer is:
vestry
'where ministers change' is the definition.
(room where church ministers change clothes)
'top lines' is the wordplay.
'top' becomes 'vest' (I've seen this before).
'lines' becomes 'ry' (abbreviation for railway).
'vest'+'ry'='VESTRY'
(Other definitions for vestry that I've seen before include "Church changing room" , "Parochial meeting-room in church" , "Church sacristy" , "Storage and dressing room in a church" , "Room in a church where sacred vessels and vestments are kept" .)