Melt old penny I found with small crack (8)
I believe the answer is:
dissolve
'melt' is the definition.
(I know that melt can be written as dissolve)
'old penny i found with small crack' is the wordplay.
'old penny' becomes 'd'.
'found with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'crack' becomes 'solve' (crack can mean to solve or decipher).
'd'+'i'+'s'+'solve'='DISSOLVE'
(Other definitions for dissolve that I've seen before include "Annul (a marriage, etc)" , "Liquify, melt" , "Pass into a solution" , "Finish" , "Change into liquid" .)