This uses deletion - a technique where you remove one or more letters from a word or phrase to create the answer.
Look for a word that means 'Reform UK, say' - think about what type of organization Reform UK is. Then remove the 'head' (first letter) from that word.
Answer: ARTY | Reform UK is a political party (PARTY). Remove the head letter 'P' to get ARTY, meaning affected or pretentious.