Marking certainly stops poet working (7)
I believe the answer is:
eyespot
'marking' is the definition.
(marking on some animals)
'certainly stops poet working' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'stops' is an insertion indicator.
'working' indicates an anagram.
'poet' anagrammed gives 'epot'.
'yes' put into 'epot' is 'EYESPOT'.
(Other definitions for eyespot that I've seen before include "Light-sensitive patch on a starfish and others" , "Cereal fungal disease" , "Plant disease" , "on a butterfly's wing" , "Evidence of disease" .)