### Clue: Pair accepts ongoing army training to begin with for Thames battle (4,4)
### Answer: BOAT RACE 
 ### Explanation: Definition: Thames battle
 BRACE(a pair of something, typically bird or animals killed in hunting) **containing**(accepts) **1st letters, respectively, of**(… to begin with) "ongoing army training".
Defn: …, specifically the competition on the Thames river between the Oxford and Cambridge rowing teams.