In comparatively stern pursuit? (5)
I believe the answer is:
after
'in comparatively' is the definition.
I can't judge whether this defines the answer.
'stern pursuit?' is the wordplay.
'stern' becomes 'aft' (similar in meaning).
'pursuit?' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'aft'+'er'='AFTER'
Can you help me to learn more?
(Other definitions for after that I've seen before include "Following, consequent to" , "Pursuing" , "Subsequently" , "back along > deck" , "In the time following" .)