Strange angel guarding one is cordial (6)
I believe the answer is:
genial
'cordial' is the definition.
(I know this)
'strange angel guarding one' is the wordplay.
'strange' is an anagram indicator.
'guarding' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'angel' anagrammed gives 'genal'.
'genal' placed around 'i' is 'GENIAL'.
'is' is the link.
(Other definitions for genial that I've seen before include "'Affable, warm and friendly (6)'" , "Cordial" , "Cheerful, affable" , "Angel I found being affable" , "Friendly and agreeable" .)