Promise son clothes (5)
I believe the answer is:
swear
'promise' is the definition.
(as in swearing something is true)
'son clothes' is the wordplay.
I cannot really understand how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is found in the answer.
'clothes' could be 'wear' (synonyms) and 'wear' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Eff and blind" , "Curse" , "Avow" , "Asseverate" , "Vow; blaspheme" .)