We must keep one little coin clean (4)
I believe the answer is:
wipe
'clean' is the definition.
(thesaurus)
'we must keep one little coin' is the wordplay.
'must keep' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'little coin' becomes 'p' (abbreviation for pence).
'i'+'p'='ip'
'we' enclosing 'ip' is 'WIPE'.
(Other definitions for wipe that I've seen before include "Swab" , "Clean with a damp cloth" , "use cloth" , "Clean (a surface)" , "Push cloth over" .)