Old man's time is tense (4)
I believe the answer is:
past
'is tense' is the definition.
(I've seen this before)
'old man's time' is the wordplay.
'old man's' becomes 'pas' (both can mean someone's father).
'time' becomes 't' (abbreviation).
'pas'+'t'='PAST'
(Other definitions for past that I've seen before include "Yesteryear" , "tense" , "A person's earlier life history" , "Days gone by" , "Beyond" .)