Pen capturing the Guardian's spirit (7)
I believe the answer is:
courage
'spirit' is the definition.
(I've seen this before)
'pen capturing the guardian's' is the wordplay.
'pen' becomes 'cage' (I've seen this before).
'capturing' means one lot of letters goes inside another.
'the guardian's' becomes 'our' (I've seen this in other clues).
'cage' enclosing 'our' is 'COURAGE'.
(Other definitions for courage that I've seen before include "Daring" , "Gallantry" , "Bravery, spirit" , "Valour" , "Bravery, pluck" .)