Takes in sustenance from nipples, first off (4)
I believe the answer is:
eats
'takes in' is the definition.
('eat' can be a synonym of 'take in')
'nipples first off' is the wordplay.
'nipples' becomes 'teats' ('teat' can be a synonym of 'nipple').
'first off' means to remove the first letter.
'teats' with its initial letter removed is 'EATS'.
'sustenance from' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for eats that I've seen before include "Consumes or dines" , "corrodes" , "fare" , "Swallows food" , "Takes food" .)