ok... if mainly required for monitoring, guitar pocket headphone amps...
a pair of discrete ear bud headphones and off you go...

fair few available, maybe start off looking at these..

Vox amPlug Headphone Guitar Amplifier - AC30

29 quid at Amazon

Very popular - these plug direct into the instrument jack socket,
so a male to female converter lead would be required if you need to hide it away in a pocket.

More Headphone amps ..

http://www.amazon.co.uk/s/ref=nb_sb_noss_2?url=search-alias%3Dmi&field-keywords=guitar%20headphone