Have trouble walking labradors if mutts pull leads (4)

I believe the answer is:

limp

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'have trouble walking' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'labradors if mutts pull leads' is the wordplay.
'leads' indicates taking the first letters.
The initial letters of 'labradors if mutts pull' is 'LIMP'.

Can you help me to learn more?

(Other definitions for limp that I've seen before include "Hanging loosely - hobble" , "Weak" , "Lacking stiffness" , "Feeble" , "Uneven manner of walking" .)

I've seen this clue in the Sydney Morning Herald.

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