One entering through wharf (4)
I believe the answer is:
pier
'wharf' is the definition.
(I know that wharf can be written as pier)
'one entering through' is the wordplay.
'one' becomes 'I' (Roman numeral).
'entering' indicates putting letters inside.
'through' becomes 'per' ('per' can mean through or by means of).
'i' put inside 'per' is 'PIER'.
(Other definitions for pier that I've seen before include "Breakwater; arch pillar" , "Seaside resort feature" , "One jutting into sea" , "A landing stage or promenade" , "Structure extending seawards" .)