Seriousness of life once besieged by almost complete apprehension (8)
I believe the answer is:
gravitas
'seriousness' is the definition.
(I've seen this before)
'life once besieged by almost complete apprehension' is the wordplay.
'life once' becomes 'avit' (I can't explain this - if you can you should give a lot more credence to this answer).
'besieged by' indicates putting letters inside (inserted letters are surrounded or besieged).
'almost complete' means to remove the last letter (most but not all of the word).
'apprehension' becomes 'grasp' (grasp is a kind of apprehension).
'grasp' with its final letter taken off is 'gras'.
'avit' placed into 'gras' is 'GRAVITAS'.
'of' is the link.
Can you help me to learn more?
(Other definitions for gravitas that I've seen before include "Seriousness - sobriety" , "Dignified manner/presence" , "Formal bearing" , "Dignity of manner" , "Weight" .)