I left, determined to block messenger in mission (10)
I believe the answer is:
pilgrimage
'mission' is the definition.
(thesaurus)
'i left determined to block messenger' is the wordplay.
'i left' becomes 'il' (this could be a standard abbreviation I've not previously seen).
'determined' becomes 'grim' (as in doing something with grim determination).
'to block' means one lot of letters goes inside another (block can mean to fill a hole in something).
'messenger' becomes 'page' (page is a kind of messenger).
'il'+'grim'='ilgrim'
'ilgrim' going within 'page' is 'PILGRIMAGE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for pilgrimage that I've seen before include "Religiously inspired journey" , "journey to shrine" , "Journey to a sacred place" , "special journey?" , "Religiously-inspired journey" .)