Sharpen by putting each end in water (4)
I believe the answer is:
whet
'sharpen by' is the definition.
'whet' can be an answer for 'sharpen' (whetting is a kind of sharpening). I am not sure about the 'by' bit.
'each end in water' is the wordplay.
'end' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'water' becomes 'wet' (I can't justify this - if you can you should believe this answer much more).
The final letter of 'each' is 'h'.
'h' placed into 'wet' is 'WHET'.
'putting' acts as a link.
Can you help me to learn more?
(Other definitions for whet that I've seen before include "Excite" , "appetizer" , "Sharpen - stimulate" , "Make more acute" , "Sharpen by grinding" .)