A bridge in Trim that is not high enough (3,5)

I believe the answer is:

cut short

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'trim that is not high enough' is the definition.
'cut short' can be an answer for 'trim' (thesaurus). I'm not sure about the remainder of the definition.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for cut short that I've seen before include "Stopped talking" , "instruction to army barber" , "ended prematurely" , "Interrupted" , "Abbreviate" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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