Stan's deaf - Tess possibly shows loyalty (13)
I believe the answer is:
steadfastness
'loyalty' is the definition.
(steadfastness is a kind of loyalty)
'stan's deaf tess possibly' is the wordplay.
'possibly' is an anagram indicator.
'stans'+'deaf'+'tess'='stansdeaftess'
'stansdeaftess' with letters rearranged gives 'STEADFASTNESS'.
'shows' acts as a link.
(Other definitions for steadfastness that I've seen before include "Firm unwavering quality" , "Resolution" , "Resolute attitude" , "Loyalty" .)