Promise son to be in (5)
I believe the answer is:
swear
'promise' is the definition.
(as in swearing something is true)
'son to be in' is the wordplay.
I cannot quite see how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is present in the answer.
The remaining letters 'wear' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Use bad language" , "Affirm" , "Assert" , "Avow" , "Solemnly promise" .)