One inside for every wharf (4)
I believe the answer is:
pier
'wharf' is the definition.
(I know that wharf can be written as pier)
'one inside for every' is the wordplay.
'one' becomes 'i' (Roman numeral).
'inside' is an insertion indicator.
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'i' inserted into 'per' is 'PIER'.
(Other definitions for pier that I've seen before include "Buttress; landing place" , "Seaside structure" , "Platform out to sea" , "Break-water or landing stage" , "Structure extending seawards" .)