This is a confused attempt to combine the word 'safe' with the phrase 'nice one'. Usually blurted out in pressure situations in the company of intimidating individuals.
An almalgamation of the phrases 'safe' and 'nice one' this common used phrase has it's roots in Cambridgesurburbia, but is found in all areas of the UK
Tali: "I did like a million pieces of work today!"
Tom: "Safe one babes"