Condition of premature baby: wrinkles between sides of head (9)
I believe the answer is:
earliness
'condition of premature baby' is the definition.
(being born too early)
'wrinkles between sides of head' is the wordplay.
'wrinkles' becomes 'lines' ('line' can be a synonym of 'wrinkle').
'between' indicates putting letters inside.
'sides of head' becomes 'ears' (ears are at the sides of the head).
'lines' going inside 'ears' is 'EARLINESS'.
(Other definitions for earliness that I've seen before include "Ere snails? (Anag)" , "being in good time" .)