Lord’s wife and son are unknown (4)
I believe the answer is:
lady
'lord's wife' is the definition.
(I've seen this in another clue)
'son are unknown' is the wordplay.
'son' becomes 'lad' (lad can mean a boy or son).
'are unknown' becomes 'y' (algebra).
'lad'+'y'='LADY'
'and' acts as a link.
(Other definitions for lady that I've seen before include "Gentlewoman" , "Luck, for example" , "See 7" , "mature woman" , "Noble" .)