Side set back, fielding captain may employ it (3,4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Reversal of ( … back ) [ PART(an example of which is the left or right side of, say, an object) + GEL(to set/to solidify) ]. Answer: In cricket, a group of fielders placed close to the wicket on the leg side.