For this reason, see guards left alone (6)
I believe the answer is:
solely
'alone' is the definition.
(I know that alone can be written as solely)
'for this reason see guards left' is the wordplay.
'for this reason' becomes 'so' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'see' becomes 'ely' (see is a place with a cathedral - e.g. Ely).
'guards' is an insertion indicator (inserted letters are guarded).
'left' becomes 'l' (common abbreviation).
'so'+'ely'='soely'
'soely' placed around 'l' is 'SOLELY'.
Can you help me to learn more?
(Other definitions for solely that I've seen before include "alone" , "Exclusively, only" , "nowhere else?" , "Singly" , "Singularly" .)