Although adults loathe babies, everyone is tiny to begin with (6)
I believe the answer is:
albeit
'although' is the definition.
(I've seen this before)
'adults loathe babies everyone is tiny to begin with' is the wordplay.
'to begin with' indicates taking the first letters.
The initial letters of 'adults loathe babies everyone is tiny' is 'ALBEIT'.
(Other definitions for albeit that I've seen before include "In spite of the fact that" , "A literary although" , "'Even if, although (6)'" , "Conjunction meaning although" , "Although, inasmuch as" .)