Senior citizen’s long-standing form of rest (7)
I believe the answer is:
oldster
'senior citizen's' is the definition.
(synonyms)
'long-standing form of rest' is the wordplay.
'long-standing' becomes 'old' (similar in meaning).
'form of' indicates anagramming the letters (the letters need to be in another 'form').
'rest' anagrammed gives 'ster'.
'old'+'ster'='OLDSTER'
(Other definitions for oldster that I've seen before include "Wrinkly" , "Person getting on" , "one getting on" , "Veteran" , "One no longer inexperienced" .)