Too low to be easily grasped? (4)

I believe the answer is:

deep

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'too low' is the definition.
'deep' can be an answer for 'low' (similar in meaning). I am not sure about the 'too' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for deep that I've seen before include "late" , "Profound - engrossed" , "greatly involved?" , "Extending far down" , "down a long way" .)

I've seen this clue in the Evening Standard.

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