Possible Clues:

• Innocent as an empty gallery?
• The ingenuous painter seems out of work
• Like Eliza Doolittle or Professor 'iggins, so to speak
• Natural - uncontrived
• Frank's no oil painting!
• Sincere
• A gallery under refurbishment could be that simple
• Simple cargo for vessel travelling northwards
• Frank is ruthless for EastEnders
• Simple description of Higgins, as Eliza would say