The mutt ran when freed, the little pet (7)

I believe the answer is:

tantrum

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'little pet' is the definition.
'tantrum' can be an answer for 'pet' (I've seen this before). I'm unsure of the 'little' bit.

'the mutt ran when freed' is the wordplay.
'when freed' indicates anagramming the letters.
'mutt'+'ran'='muttran'
'muttran' is an anagram of 'TANTRUM'.

'the' is the link.

Can you help me to learn more?

(Other definitions for tantrum that I've seen before include "Pet, temper" , "Spoilt brat's fit" , "Fit - pet" , "Burst of temper" , "Frenzy" .)

I've seen this clue in the Evening Standard.

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