Saint Mark is severe (6)
I believe the answer is:
strait
'severe' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'saint mark' is the wordplay.
'saint' becomes 'st'.
'mark' becomes 'rait' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'st'+'rait'='STRAIT'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for strait that I've seen before include "Hardship - narrow passage of water" , "Sound" , "Difficulty - strict" , "watery scene?" , "Narrow waterway" .)