The term 'hackle' can refer to a few different things. In the context of a crossword, it is likely referring to a tool used in fly fishing. A hackle is a COMB-like tool used to prepare feathers for tying flies. The four letter word 'HACK' is likely referring to the act of using the hackle to COMB the feathers. The four letter word 'COMB' is likely referring to the tool itself. Finally, the seven letter word 'FEATHER' is likely referring to the material being prepared with the hackle.