Very exposed lines become worn down (5)
I believe the answer is:
erode
'become worn down' is the definition.
(I've seen this in another clue)
'very exposed lines' is the wordplay.
'exposed' says to take the centre (outside letters removed).
'lines' becomes 'ode' (lines of poetry).
The central letters of 'very' are 'er'.
'er'+'ode'='ERODE'
(Other definitions for erode that I've seen before include "Grind down" , "Gradually wear away" , "Wear away with weather" , "'Deteriorate, wear away (5)'" , "go to pieces!" .)