Are old ones tense? (5)
I believe the answer is:
exist
'are' is the definition.
('they are' means 'they exist')
'old ones tense?' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'ones' becomes 'Is' (I is the Roman numeral for one).
'tense?' becomes 't' (abbreviation used in many dictionaries).
'ex'+'is'+'t'='EXIST'
(Other definitions for exist that I've seen before include "Be alive" , "Subsist" , "Have objective reality" , "Scrape by" , "quick!" .)