Seatrain regularly displays an item of clothing (4)
I believe the answer is:
sari
'clothing' is the definition.
(sari is a kind of clothing)
'seatrain regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'seatrain' are 'SARI'.
'displays an item of' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sari that I've seen before include "Traditional Indian women's dress" , "Hindu woman's dress" , "Indian robe" , "Indian costume" , "Traditional dress of Hindu women" .)