### Clue: Hot or freezing, Papa, you must put that on! (7)
### Answer: POPULAR 
 ### Explanation: Definition: Hot
 POP(a term, like Papa, for one's father) + U("you", as in mobile phone texting) **plus**(must put that on) LAR(acronym for Liquid Argon, which is a sub-zero;freezing substance – I'm not sure about this, as it is an uncommon acronym).
Defn: In high demand;in fashion.