Son promises to get teacher's approval for concealed weapon (10)
I believe the answer is:
swordstick
'concealed weapon' is the definition.
'swordstick' can be an answer for 'weapon' (I have seen 'Hidden weapon' mean 'swordstick' so perhaps 'weapon' could also mean 'swordstick'). I am unsure of the 'concealed' bit.
'son promises to get teacher's approval' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'promises' becomes 'words' (word is a kind of promise).
'to get' means one lot of letters go next to another.
'teacher's approval' becomes 'tick' (I can't explain this - if you can you should give a lot more credence to this answer).
's'+'words'+'tick'='SWORDSTICK'
'for' is the link.
Can you help me to learn more?
(Another definition for swordstick that I've seen is " Hidden weapon".)