Cut line inside window frame (5)
I believe the answer is:
slash
'cut' is the definition.
(I know that cut can be written as slash)
'line inside window frame' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'inside' indicates putting letters inside.
'window frame' becomes 'sash' (I've seen this in another clue).
'l' going within 'sash' is 'SLASH'.
(Other definitions for slash that I've seen before include "Lacerate" , "Drastically prune" , "Cut with sweeping strokes" , "Punctuation mark" , "something between a month and a year" .)