Seem too tired to keep up (3)
I believe the answer is:
sag
'up' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how they can define each other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sag that I've seen before include "Drop" , "Droop in the middle - it's gas" , "Sink (under load)" , "Sink under weight" , "Sinking" .)