Played the part of a cadet at sea (5)
I believe the answer is:
acted
'played the part' is the definition.
(I've seen this in another clue)
'cadet at sea' is the wordplay.
'at sea' is an anagram indicator.
'cadet' is an anagram of 'ACTED'.
'of a' is the link.
(Other definitions for acted that I've seen before include "Behaved or performed" , "Trod the boards" , "took part in simulation" , "Feigned" , "Behaved like thespian" .)