You might be old (4)
I believe the answer is:
thou
'you' is the definition.
(I've seen this before)
'old' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (abbreviation) and 'o' is located in the answer.
The remaining letters 'thu' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'might be' is the link.
Can you help me to learn more?
(Other definitions for thou that I've seen before include "'Holier than - - '" , "You (obs.)" , "You, poetically of old" , "Archaic pronoun" , "Measure" .)