National Archives at Kew have the ships logs.
You can't see them online, but the link at the bottom will get you the references for Cleopatra wartime service.
What this does is search the Catalogue at national archives http://www.nationalarchives.gov.uk
Go to Search the catalogue
Input search term "Cleopatra", (don't use HMS)
then" range of dates"
then input Departmental Code "ADM"
Rather than search the National Archive, if you can find a library that has wartime AFOs (Admiralty Fleet Orders), they list all awards of decorations including MIDs. Failing that, write to the Naval Historical Branch or Naval Library in Pompey Dockyard. I have vague memories of a book called "Seedys" medal lists, does anyone else know of it ?
According to the London Gazette online, Yeoman of Signals Frederick Horace Thorpe, C/J 110249 was gazetted on 8 Sep 1942 for the award of a posthumous Mention in Despatches (MID) (link). He was among many other personnel of several ships listed, including HMS CLEOPATRA, who were decorated:
London Gazette Issue number 35695 dated 4 September 1942 said:
...for gallantry, skill and seamanship in a brilliant action against strong enemy forces which were driven off and severely damaged. This action resulted in the safe passage to Malta of an important convoy. (link)
The RN Museum Library and the Naval Historical Branch (contact details), both located in Portsmouth Naval Base, hold copies of 'Seedies' which lists decorations awarded to naval personnel up to 1958 but it is unlikely to contain any more detail than this. In the unlikely event that an individual citation survives, it will probably be held by Naval Historical Branch or the National Archives at Kew.
In a nutshell, his name was mentioned in the dispatch sent to the head shed for something he had done which was worthy of special praise. He would have been awarded with an oak leave type thingy on his medal ribbon.