Mature lone pirate held up harbours (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'lone pirate held up harbours' is the wordplay.
'harbours' says the answer is hidden in the clue.
'held up' says the letters should be written in reverse.
'nepir' is hidden within 'lone pirate'.
'nepir' back-to-front is 'RIPEN'.
(Other definitions for ripen that I've seen before include "Get ready for 2" , "Become ready to eat" , "Grow older" , "Become ready for picking" , "Become ready for harvesting" .)