Sentence:  Alfie was familiar with the term ‘<WSD>understrapper</WSD>'. The Harry Fielding Alfie knew was an <WSD>understrapper</WSD>; a bob-a-job man for MI5. Somebody who could do a bit of house- or office-breaking, or some surveillance work. 
Meaning of the word marked with <WSD> tags:  A freelance operator for MI5.