Languishing? Nothing so weak, ultimately (8)
I believe the answer is:
lovesick
'languishing?' is the definition.
(thesaurus)
'nothing so weak ultimately' is the wordplay.
'nothing' becomes 'love' (love is zero in tennis).
'so' becomes 'sic' ('sic' is 'so' in Latin).
'ultimately' suggests the final letters.
The last letter of 'weak' is 'k'.
'love'+'sic'+'k'='LOVESICK'
(Other definitions for lovesick that I've seen before include "Pining for one's sweetheart" , "Nauseatingly infatuated?" .)