One might be in the pit as a Little Mix musical starts in tune (6,6)

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

A charade of GREASE (‘musical’, stage and film) plus M (‘a little M ix’) plus ON KEY (‘in tune’), with ‘starts’ indicating the order of the particles. In the definition, the ‘pit’ might be in a body shop.