Piratical sea dog? (5)
I believe the answer is:
rover
'sea dog?' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for rover that I've seen before include "gypsy" , "Vehicle driven remotely over extraterrestrial terrain" , "Wanderer, archery mark" , "Wanderer, traveller" , "Person given to wandering" .)