### Clue: One could be responsible for 13, with a tense pig in sty (8)
### Answer: PATHOGEN 
 ### Explanation: Definition: One could be responsible for 13
  [A + T(abbrev. for "tense" in linguistics) + HOG(a pig)] **contained in**(in) PEN(an enclosure,sty). 
Answer: A disease-causing agent, especially a bacterium, virus or other micro-organism, one that could be responsible for swine flu(answer to 13across).