Journey taken for a very good reason (10)
I believe the answer is:
pilgrimage
'journey' is the definition.
(I know that pilgrimage is a type of journey)
'taken for a very good reason' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'good' could be 'pi' (old-fashioned term for pious) and 'pi' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pilgrimage that I've seen before include "special journey?" , "journey to shrine" , "Journey to a sacred place" , "Holy journey" , "What Chaucer,s travellers were engaged upon" .)