Second of pirates person at sea enlists (5,2)
I believe the answer is:
ropes in
'enlists' is the definition.
'second of pirates person at sea' is the wordplay.
'second of pirates' becomes 'i' (2nd letter of 'pirates').
'at sea' indicates an anagram.
'i'+'person'='iperson'
'iperson' with letters rearranged gives 'ROPES IN'.
(Other definitions for ropes in that I've seen before include "Gets involved" , "Recruits" , "Involves" .)