Investing euros originally, Welshman’s money is soon spent! (10)
I believe the answer is:
evanescent
'soon spent' is the definition.
(evanescent can mean used up or passing quickly)
'investing euros originally welshman's money' is the wordplay.
'investing' indicates putting letters inside (inserted letters are taken in or invested).
'originally' suggests taking the first letters.
'welshman's' becomes 'Evan's' (common Welsh name).
'money' becomes 'cent' (eg denomination of dollars).
The initial letter of 'euros' is 'e'.
'e' going within 'evans' is 'evanes'.
'evanes'+'cent'='EVANESCENT'
'is' is the link.
(Other definitions for evanescent that I've seen before include "evaporating slowly" , "Disappearing" , "Transient" , "not permanent" , "Passing" .)