Unchanging exterior masking sign of affection (7)
I believe the answer is:
eternal
'unchanging' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for eternal that I've seen before include "Never-ending, existing always" , "Everlasting, not affected by time" , "One being forever" , "Lasting forever, tediously persistent" , "like Rome?" .)