Medicine that's too nice not to finish (5)
I believe the answer is:
tonic
'medicine' is the definition.
(I know that tonic is a type of medicine)
'too nice not to finish' is the wordplay.
'too' becomes 'to' (I can't justify this - if you can you should believe this answer much more).
'not to finish' means to remove the last letter.
'nice' with its final letter removed is 'nic'.
'to'+'nic'='TONIC'
'that's' is the link.
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "something invigorating" , "that may be mixed with gin" , "something providing a boost" , "__water; gin mixer" , "shot in the arm" .)