Sentence:  So I went and laid down on the grass. While laying there I piped a reeler whom I knew. He had a <WSD>nark</WSD> (a policeman's spy) with him. So I went and looked about for my two pals, and told them to look out for F. and his <WSD>nark</WSD>. 
Meaning of the word marked with <WSD> tags:  A police spy or informer.