Cut a corner in a window frame (5)
I believe the answer is:
slash
'cut' is the definition.
(I know that slash is a kind of cut)
'a corner in a window frame' is the wordplay.
'a corner' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'a window frame' becomes 'sash' (I've seen this before).
'l' going inside 'sash' is 'SLASH'.
Can you help me to learn more?
(Other definitions for slash that I've seen before include "Cut; reduce drastically" , "Punctuation mark" , "Cut with sweeping strokes" , "Greatly reduce (prices)" , "Make sharp, long cut" .)