Clue: Pleased to get one lariat designed to catch old combatant (12)
Answer: GLADIATORIAL
Explanation: Definition: ombatant 
An envelope ('to catch') of O ('old') in GLAD ('pleased') plus I ('one') plus ATRIAL, an anagram ('designed') of 'lariat'. The definition 'combatant' is more commonly used as a noun, but it can be an adjective.