Relation of pleasant Eastern content (5)
I believe the answer is:
niece
'relation' is the definition.
(niece is a kind of relation)
'pleasant eastern content' is the wordplay.
'pleasant' becomes 'nice' ('nice' is associated in meaning with 'pleasant').
'eastern' becomes 'e'.
'content' indicates putting letters inside.
'nice' going around 'e' is 'NIECE'.
'of' is the link.
(Other definitions for niece that I've seen before include "Young female relative" , "A sibling's daughter" , "Family relation" , "Brother's daughter" , "A younger female relative" .)