Mark is always very good on Monday (3)
I believe the answer is:
aim
'mark' is the definition.
(thesaurus)
'always very good on monday' is the wordplay.
I cannot quite see how this works, but
'monday' could be 'm' and 'm' is found in the answer.
'very good' could be 'ai' (resembles 'A1', old-fashioned term for excellent) and 'ai' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for aim that I've seen before include "Goal, objective" , "Train" , "Point towards a target, or have ambition" , "Direct, intend" , "Steady, ... fire" .)