What babies need: rest before food (7)
I believe the answer is:
nappies
'what babies need' is the definition.
(babies wear nappies)
'rest before food' is the wordplay.
'rest' becomes 'nap' (a nap is a short sleep).
'before' means one lot of letters go next to another.
'food' becomes 'pies' (pies are a type of food).
'nap'+'pies'='NAPPIES'
(Other definitions for nappies that I've seen before include "Baby's pads" , "What Americans call diapers" , "Covers" , "Cloths for babies" , "They're diapers in America" .)