Call into question work finally undertaken by dog (6)
I believe the answer is:
oppugn
'call into question' is the definition.
(I've seen this before)
'work finally undertaken by dog' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'finally' indicates one should take the final letters.
'by' says to put letters next to each other.
'dog' becomes 'pug' (pug is a kind of dog).
The last letter of 'undertaken' is 'n'.
'n' put after 'pug' is 'pugn'.
'op'+'pugn'='OPPUGN'
(Other definitions for oppugn that I've seen before include "Dispute truth or validity of" , "Question the validity of (something)" , "attack" , "Call into question" .)