Prior to unit getting broken on line (5)
I believe the answer is:
until
'prior to' is the definition.
(I've seen this in another clue)
'unit getting broken on line' is the wordplay.
'getting broken' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'line' becomes 'l' (used when specifying particular lines from a poem).
'unit' with letters rearranged gives 'unti'.
'unti'+'l'='UNTIL'
(Other definitions for until that I've seen before include "Not beyond (hour)" , "Unlit (anag.)" , "Up to that moment" , "As far as" , "Up to (a point in time)" .)