Maybe his or her tense successor (5)
I believe the answer is:
their
'maybe his or her' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'tense successor' is the wordplay.
'tense' becomes 't' (grammatical abbreviation).
'successor' becomes 'heir' (synonyms).
't'+'heir'='THEIR'
Can you help me to learn more?
(Other definitions for their that I've seen before include "Relating to them" , "Belonging to those people" , "Possessive pronoun" , "Belonging to him (perhaps wrongly)" , "belonging to an individual" .)