Thousand pounds in good currency (5)
I believe the answer is:
grand
'thousand pounds' is the definition.
'good currency' is the wordplay.
'good' becomes 'g' (abbreviation).
'currency' becomes 'rand' (I've seen this before).
'g'+'rand'='GRAND'
'in' acts as a link.
(Other definitions for grand that I've seen before include "A slangy PS1,000" , "1,000 GBP" , "Large and imposing" , "Splendid - piano" , "GBP1,000" .)