Crossword Genius

Seem too tired to keep up (3)

Ross

I believe the answer is:

sag

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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