Like Liz Taylor or James Dean, one's not featured in main sequence (5,4)

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

I am not sure of the parsing for this.  It may be a reference to GIANT STA i R s (for GIANT ‘s Causeway in Northern Ireland near the sea [main]) excluding (not) I (one) to form GIANT STAR, but this doesn’t seem quite right to me