Caravan mostly reputed to carry eastern king (5)
I believe the answer is:
serai
'caravan' is the definition.
(another word for caravanserai)
'mostly reputed to carry eastern king' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'reputed' becomes 'said' (said to be, reputed to be).
'to carry' is an insertion indicator.
'eastern' becomes 'e'.
'king' becomes 'R' (abbreviation of Latin rex).
'said' with its final letter taken away is 'sai'.
'e'+'r'='er'
'sai' placed around 'er' is 'SERAI'.
(Other definitions for serai that I've seen before include "Desert inn; raise (anag.)" , "Eastern inn" , "Khan" , "it'll refresh in the desert" .)