Seem too tired to do a holdup? (3)

I believe the answer is:

sag

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'seem' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sag that I've seen before include "Sink under weight" , "Slump" , "Drop" , "Sink (under load)" , "Subsidence" .)

I've seen this clue in the Evening Standard.

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