Elizabethan navigator finally turned libertine (5)
I believe the answer is:
drake
'elizabethan navigator' is the definition.
(Sir Francis Drake)
'finally turned libertine' is the wordplay.
'finally' indicates one should take the final letters.
'libertine' becomes 'rake' (rake is a kind of libertine).
The final letter of 'turned' is 'd'.
'd'+'rake'='DRAKE'
(Other definitions for drake that I've seen before include "Elizabethan circumnavigator" , "Male water-bird" , "world traveller" , "First Englishman to sail round the world" , "First English captain to sail round the world" .)