Steadfast in affection or allegiance (8)
I believe the answer is:
faithful
'steadfast in affection or allegiance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for faithful that I've seen before include "Loyal and steadfast" , "Steadfast in affection or allegiance" , "Trustworthy and true" , "Loyal and true" , "Steadfast and true" .)