Accept pet auk must be replaced (4,2)
I believe the answer is:
take up
'accept' is the definition.
(taking up is a kind of accepting)
'pet auk must be replaced' is the wordplay.
'must be replaced' is an anagram indicator.
'pet'+'auk'='petauk'
'petauk' with letters rearranged gives 'TAKE-UP'.
(Other definitions for take up that I've seen before include "Acceptance of something offered" , "Accept" , "lift" , "Adopt (pastime) - shorten (garment) - absorb (liquid)" .)