Inappropriate force will break one (5)
I believe the answer is:
unfit
'inappropriate' is the definition.
(I've seen this in another clue)
'force will break one' is the wordplay.
'force will' becomes 'f' (I can't explain this - if you can you should give a lot more credence to this answer).
'break' means one lot of letters goes inside another.
'one' becomes 'unit' (synonyms).
'f' inserted within 'unit' is 'UNFIT'.
Can you help me to learn more?
(Other definitions for unfit that I've seen before include "Not in good physical condition, or not suitable" , "Not in good shape physically" , "Inapt" , "Not in good physical shape (5)" , "Ill - out of shape" .)