It ultimately gets old (3)
I believe the answer is:
sex
'it' is the definition.
('it' is a common euphemism for sex)
'ultimately gets old' is the wordplay.
'ultimately' suggests the final letters.
'old' becomes 'ex' (prefix meaning former or onetime).
The last letter of 'gets' is 's'.
's'+'ex'='SEX'
(Other definitions for sex that I've seen before include "Male or female" , "This symbol is epitome of glamour" , "Hanky-panky" , "Gender" , "Expert in the treatment of coital problems" .)