Exulted when, given credit, one was in debt (6)
I believe the answer is:
crowed
'exulted' is the definition.
(crow can mean boast of or exult in)
'given credit one was in debt' is the wordplay.
'given' means one lot of letters go next to another.
'credit' becomes 'cr' (abbreviation).
'one was in debt' becomes 'owed' ('they owed' means 'they were in debt').
'cr'+'owed' is 'CROWED'.
'when' acts as a link.
(Other definitions for crowed that I've seen before include "Boasted" , "was triumphant" .)