Clue: Add 1/2 of 12 to 1/4 of 19 with confidence (6)
Answer: SURELY
Explanation: Add 1/2 of 12 to 1/4 of 19 with confidence (6)
Relates to parts of the answers to 12 ac and 19dn, so… 
…(trea)SURE + (modest)LY