Clue: Providing energy, dainty sweetheart for 24? (1,4,4)
Answer: I FEEL FINE
Explanation: Definition: 24
*I Feel Fine* was a 'number one'/chart-topping hit for the Beatles. 24ac is NO ONE=No. 1='number one' 
IF="Providing" + E (energy) + ELFIN="dainty" + E="heart" of sw-E-et