Impressive amount of money (5)
I believe the answer is:
grand
This clue is a double definition.
'impressive' is the first definition.
(similar in meaning)
'amount of money' is the second definition.
(a thousand pounds)
(Other definitions for grand that I've seen before include "Large and imposing" , "Imposing; PS1,000" , "sum of money" , "principal" , "PS1k" .)