Ken changed two pounds for toll (5)
I believe the answer is:
knell
'toll' is the definition.
(thesaurus)
'ken changed two pounds' is the wordplay.
'changed' is an anagram indicator.
'two' indicates the doubling of some letters.
'pounds' becomes 'l' (L can represent the monetary pound).
'l' doubled is 'll'.
'ken' is an anagram of 'kne'.
'kne'+'ll'='KNELL'
'for' is the link.
(Other definitions for knell that I've seen before include "''The curfew tolls the ... of parting day'' (Thomas Gray)" , "Toll (bell)" , "Funereal bell sound" , "Awesome ring" , "Sound of tolling bell" .)