Five pence out of pocket under a guarantee that’s expired (6)
I believe the answer is:
avouch
'a guarantee that's expired' is the definition.
(archaic word meaning to guarantee)
'five pence out of pocket under' is the wordplay.
'five' becomes 'av' (I can't justify this - if you can you should give a lot more credence to this answer).
'pence' becomes 'p'.
'out of' indicates named letters should be taken away.
'pocket' becomes 'pouch' (synonyms).
'under' says to put letters next to each other.
'pouch' with 'p' removed is 'ouch'.
'av'+'ouch'='AVOUCH'
(Other definitions for avouch that I've seen before include "Assert (archaic)" , "Assert; guarantee" .)