Ruin regularly leads writer to mature (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'ruin regularly leads writer' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'leads' is a charade indicator (letters next to each other) (some letters go in front of others).
'writer' becomes 'pen' (something which writes).
The alternate letters of 'ruin' are 'ri'.
'ri'+'pen'='RIPEN'
'to' is the link.
(Other definitions for ripen that I've seen before include "Mature, develop" , "Season" , "Get ready for 2" , "Make ready" , "Develop fully" .)