Twice a day, in she comes! (3)
I believe the answer is:
ada
'she comes' is the definition.
I can't judge whether this defines the answer.
'twice a day in' is the wordplay.
'twice' indicates the doubling of some letters.
'day' becomes 'd' (abbreviation for day).
'in' means one lot of letters goes inside another.
'a' duplicated is 'aa'.
'aa' enclosing 'd' is 'ADA'.
Can you help me to learn more?
(Other definitions for ada that I've seen before include "Computer language; girl's name" , "Computer lang.; f. name" , "girl?" , "- Lovelace, Byron's daughter & computer pioneer" , "Woman" .)