Moving critter to Ark — an American custom (5,2,5)
I believe the answer is:
trick or treat
'an american custom' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'moving critter to ark' is the wordplay.
'moving' is an anagram indicator (letters are moved around).
'critter'+'to'+'ark'='crittertoark'
'crittertoark' anagrammed gives 'TRICK OR TREAT'.
Can you help me to learn more?
(Other definitions for trick or treat that I've seen before include "Halloween ordeal?" , "Children's protection racket at Hallowe'en?" , "Unwelcome Halloween tradition" , "Halloween custom" , "October 31 blackmail" .)