Porcine mammal upended mushroom with a piercing yell (7)

'porcine mammal' is the definition.
(peccaries are pig-like mammals)

'upended mushroom with a piercing yell' is the wordplay.
'upended' shows that the letters should be reversed in order (in a down clue, letters go up).
'mushroom' becomes 'cep' (name for Boletus edulis - an edible mushroom).
'with' says to put letters next to each other.
'piercing' means one lot of letters goes inside another (some letters pierce a hole in others).
'yell' becomes 'cry' (synonyms).
'cep' reversed gives 'pec'.
'a' going inside 'cry' is 'cary'.
'pec'+'cary'='PECCARY'