Strive to drink decaf regularly - that's sad (5)
I believe the answer is:
teary
'sad' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'strive to drink decaf regularly' is the wordplay.
'strive' becomes 'try' (I can't explain this - if you can you should give a lot more credence to this answer).
'to drink' means one lot of letters goes inside another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'decaf' are 'ea'.
'try' going around 'ea' is 'TEARY'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for teary that I've seen before include "Weeping" , "Weepy" , "Emotional" , "Watery" , "Upset" .)