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' indicates the answer is hidden within the clue.
'held up' says the letters should be written in reverse.
'nepir' can be found hidden inside 'lone pirate'.
'nepir' backwards 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" .)