Being infirm, wane again (4)
I believe the answer is:
anew
'again' is the definition.
(synonyms)
'being infirm wane' is the wordplay.
'being infirm' indicates anagramming the letters.
'wane' is an anagram of 'ANEW'.
(Other definitions for anew that I've seen before include "Once more (poetic)" , "From the beginning again" , "Again but differently" , "Afresh" , "Wane in a different way, again" .)