Brown lead from the peculiar pet (7)
I believe the answer is:
tantrum
'pet' is the definition.
(I've seen this before)
'brown lead from the peculiar' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'lead from' says to take the initial letters (the letter that leads the word).
'peculiar' becomes 'rum' (synonyms).
The initial letter of 'the' is 't'.
'tan'+'t'+'rum'='TANTRUM'
(Other definitions for tantrum that I've seen before include "Display of bad temper" , "Childish display of temper" , "petulant outburst" , "Fit of ill-temper" , "Fit of childish temper" .)