Breakwater protects one adventurer (7)
I believe the answer is:
pioneer
'adventurer' is the definition.
(thesaurus)
'breakwater protects one' is the wordplay.
'breakwater' becomes 'pier' (I've seen this before).
'protects' is an insertion indicator (outer letters protect inside letters).
'pier' placed around 'one' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "Explorer - innovator" , "original leader" , "Ripe one (anag.)" , "Innovator; first settler" , "Innovator; early settler" .)